The top thread is really loopy on the underside of the fabric, sometimes it misses a stitch, sometimes, the upper thread breaks, and it often gets so tangled underneath that the machine won't feed.

Re-check your stitch sample. Pull on the loopy thread. Is it really the bobbin thread, or is it the top thread looping under the fabric? FWIW, stitch problems appearing under the fabric are usually top thread issues, stitch problems on top of the fabric are usually bobbin related.

You can try this on both of your machines to see if it makes a difference.

Be sure to install a brand new needle.
Remove the top thread completely from the machine.
ALWAYS RAISE the presser foot and rethread from the beginning.
Make sure the thread path is correct.
Set the top tension to the midway point and retest your machine.

If the top thread is showing under the fabric, tighten the top tension, If the bobbin thread is showing on the top of the fabric, loosen your top tension. The perfect tension is when the two threads meet in the middle of the fabric.

...

...

Be sure to Always Raise the presser foot when threading the top thread. This may help with your situation.

Look very closely at your sample stitching and determine which thread (top or bobbin) is looping and which is breaking. That will narrow down where the problem exists.

A top thread that loops and makes thread "barfs" under the fabric is generally an upper thread tension problem.
Try: Remove the top thread. ALWAYS RAISE the presser foot and rethread from the beginning. Set the upper tension to the midway point (probably will need to tweak this tension depending on your machine).

Sewing Lesson 10 How to Fix Tension on Your Sewing Machine

Understanding Thread Tension Threads

Sewing Machine Tension

Adjust your top tension if it's too tight. To decrease your top tension if it is too tight, turn your knob so the numbers are decreasing. Try ½ to 1 number lower, then test the stitches on a piece of scrap fabric. Continue until it looks even on both sides and you can no longer see the bottom thread on the top.

How to Adjust the Tension on a Sewing Machine: 13 Steps

www.wikihow.com/Adjust-the-Tension-on-a-Sewing-Machine https://www.google.co.uk/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=Sewing+Machines+Setting+tensions

First, try a brand new needle.

Second, consider using a better quality thread. By all means, AVOID old or bargain bin threads. Coats & Clark used to be a standard, but thread quality has improved over the past few years, so you may want to check out other brands like Aurafil, Superior, Isacord, etc. Also, cotton wrapped polyester thread releases thread lint that contributes to build-up of gunk in the machine. Some sewists have noted that they experience more sewing issues when using C&C threads. However, some machines will do better with certain thread brands than others, so some experimentation may be beneficial.

Third, check the stitch on your sample again. Generally, when thread loops appear on the underside of the fabric, it is the upper thread that is the problem. If loopy threads appear on the top of the fabric, it is the bobbin thread that is the issue. You can even use a different color in the top to help distinguish which thread is the problem.

Fourth, when threading the upper thread, Always Raise the Presser Foot during the threading process so the tension disk is released and the thread can seat properly. "Bird nesting" occurs when the thread is not seated in the tension disk.
Sewing Machine Thread Bunching Up Here Why

Birds nest under the fabric big loops of thread top side looks good...

Fifth, when the upper thread appears on the underside of the fabric, the upper thread tension is too loose. If the bobbin thread appears on the top of the fabric, the tension thread is too tight (or the bobbin thread is loose). Start out with the upper tension set at the midway point and tweak it from that point. The tension is ideal when both upper and bobbin threads meet in the middle of the fabric.

Also, remember that tension is not static--a tension setting for chiffon will most likely not work on cotton or denim. Get comfortable adjusting the tension to suit the project you are sewing.

..

try a new needle

try adjusting the top thread tension to see if that helps the bottom loopy stitch

Install a brand new needle. Make sure you are using thread and needle that are compatible with the fabric you are sewing.

Change the type of thread you are using. Old or bargain bin threads will frequently cause problems.

Clean the tension disks to make sure there is nothing stuck that is preventing the thread from feeding.

Remove all the thread and re-thread from the beginning. FIRST raise the presser foot so the tension disks will allow the thread to seat properly. (Consult your owner's manual.) Make sure you are threading everything in the proper sequence and not missing any steps.

Set the upper tension at the halfway point between the high and low number.

Stitch a test on a piece of your scrap fabric from your project.

The ideal tension is when the upper thread and bobbin thread meet in the middle of the fabric. If the bobbin thread is appearing at the top of the fabric, your upper tension is too tight and needs to be loosened. If the upper thread is appearing under the fabric or forming a loopy mess, the upper thread is too loose.

Keep adjusting and testing until the tension appears correct.

If the thread continues to break, then there may be a burr that is catching the thread and breaking it. Or the timing could be off. Take it for service.

http://www.schmetzneedles.com/all-about-needles/

http://www.sewingandcraftcorner.com/why-does-my-thread-keep-breaking-while-i-sew/

...

Rethread the machine. Its almost always caused by poor quality threads or misthreading.

try adjusting the top thread tension one number or letter higher
and sew on a scrap piece of fabric till the stitches look normal.

Recheck the tension on the top thread. Test it before you put it through the needle to see if there's any tension. It should pull but with some resistance. Also check the needle and make sure it is the right type for the fabric and not bent etc.

Looping on the underside of the stitch is from the upper thread, usually tension problem. But do check the bobbin tension and make sure it isn't too tight, though that's not usually the problem you're describing. The two tensions need to work together in order to get a nice, flat (not puckered or loopy) stitch.

One last thing you might check too, though again this wouldn't typically be the problem - is the dogfeed moving the material along under the presser foot correctly?

Most of the time the problem you're describing is due to not enough tension on the upper thread.