My thread slips out of the needle every time it starts to sew, if I stop the machine and pull a little bit of thread out then it does NOT slip and sews perfectly

hi there.
see if this link helps you out.
Sewing Machine Thread Bunching Up Here Why
or this one.if you go to this one and another little window will open on the page just close this one down.
Tilly and the Buttons Five Common Sewing Machine Mistakes How to Fix Them
also this one.
How to Achieve Ideal Sewing Machine Thread Tension Threads
Sewing Machines Thread build up on my finger stitch Google Search

it indicates that you have a situation called " bird nesting" and that is a result of the needle thread tension being too loose
needle tension should be tight so that when you pull the thread to thread the needle you have to exert a bit of effort to pull the thread through the tension plates
you can adjust up or down from that feeling
thread thickness affects the tension and needle eye size also affects the tension as a small eye on large thread makes the thread movement hard indicating less tension or a big eye on a small thread affects the tension as the thread slips easily and needs more tension to pull the bobbin thread up to the centre of the material being sewn

Does the top of the machine lift up ? if so, there may be a diagram under there. And you can get a manual from Sears too, which will have the directions in it. But it's not hard to thread, has a very basic threading system. Put thread on spool holder, with thread coming out from the front of the spool. Pull out a foot or so of thread, slip it into the guide at rear of machine, where the two main sections appear to join together, right at the top of the machine. Then pull thread forward from the guide, across the top of the machine. There may be a guide at the front edge of the machine top, if so, slip thread into that guide. It's been awhile so I can't recall if there is or isn't a front guide at the top. But then you go down the front of the machine, where you slip thread underneath the bottom of the section with the two vertical slots. Lift thread upward and gently over to the left 'til it catches in the hook. You can move the handwheel to move the hook if need be, it can be hard to catch if it's right at the top of it's travel. Once thread is in the hook, take the thread down to toward the needle. There's a little guide slot above the needle that you slip the thread into, just a simple wire hook mounted sideways. Slip thread into it from the open, left side, toward the right 'til it's caught, then pull down to thread needle, from the front to the back.

"Coming undone" What do you mean? Is it breaking while sewing? Is it too short and is slipping back through the needle?

If it's breaking, try changing the needle. If i's slipping back... give it some length when you thread.... and at the end of a seam Run it out... 10 inches before you cut it off close to the fabric. That little bit of thread waste will make you feel much better about your serger.

Unthread the machine fully. Remove the bobbin. Is the bobbin thread the same as the top thread? Is it a Quality thread (yes, cheapie thread can give big issues!!) Do you have several different threads wound onto the bobbin?? Is the bobbin thread wound on quite tight? If you push your finger nail into the thread wound onto the bobbin, it must not feel soft or spongy. Check if there is any roughness / burr on the any part of the shuttle hook. If so, remove it with 800 grade wet and dry abrasive or similar. Put in a NEW needle as per user manual specs (not a 'new' one you used before 'just for a little bit'.!) REPEAT - put in a NEW needle - amuse me!!. Place the bobbin into the bobbin holder (the CORRECT direction - yes there is a Right and a Wrong direction!), thread it ready to put back into the machine. BEFORE you do, hold onto the thread tail and let the bobbin and bobbin case dangle. Thread should slip thru when you give a gentle pull upwards. If no slip, loosen the tension on the tension finger a LITTLE. If it slides down easy, tighten it a little. Fit the bobbin into the machine. Lift the presser foot (this releases the top thread tension discs) Check there are no bits of thread or fluff between the tension discs Rethread the top thread - check the thread path is 100% correct. Hold onto the end of the needle thread and draw up the bobbin thread. Hold both threads, start to sew - check the result. Tip : If you use a different color bobbin thread (same brand etc etc) it will help you to see more easily what is going on. You should be seeing some of the bobbin thread color on the top side of the sewing. If not, INCREASE top tension a little - retry.If there is a lot showing, decrease top tension. Good luck

First let's make sure you threaded the upper thread properly. It MUST be threaded with the presser foot in the UP position. (this opens the tensions discs slightly and allows the thread to be seated correctly) Sew a test seem on the same type of fabric. If it is now sewing ok... you are good to go. If you still have some problems. Make sure you grasp the threads as you start a seam. Did that solve it? If not.. you'll have to adjust your top tension slowly testing the stitch after each adjustment... tighten the tension until it no longer nests underneath.

Good luck.

Is it slipping out of the needle or breaking? If it is truly slipping.... grasp the threads and hold them while you start the seam. If it is breaking at the eye of the needle change it out and make sure the new one is the proper size for your thread.

I know you've said you have checked the tensions but that usually is the problem. When you pull on the top thread after it goes through the tension but before you thread it through the needle is there some tension but not so much that it breaks when you gently pull on it? It will be a bit harder to pull it at the point after it goes through the needle but it should have a bit of give there too without breaking.

Also check the bobbin again. Take it out of the machine and examine how the thread is wound around the bobbin. Is it all criss crossed or is it fairly evenly wound? Is it in the right direction and through the tension slot? Now dangle it by the thread while in the bobbin case. It should not just reel out to the floor but should slip a bit with some tension and hang there. A slight **** of the wrist should cause it to release a bit more thread and drop a bit more to the floor. It should allow slip but it shouldn't just reel out fast.

When you reinsert the bobbin case into the machine you should feel/hear a slight click as it catches into place.

Also check the needle though that usually isn't what causes this kind of problem.

The thread will break when it has no 'give'. Either the tensions aren't quite right or it is catching somewhere along the way through the various guides where you thread it. Oh and make sure the spool itself is freely turning on the spindle! Sometimes it is because of catching at that point and simply turning the spool upside down so it can't catch will solve the problem ! :)

Suggest taking the machine back to where you bought it and have them try to help if you are doing something wrong, or replace the machine if it has an internal problem. Good luck!

No top tension, follow manual to thread top again and make sure tension dial is set to 5 and thread is pulled between the tension discs fully.