My Necchi 4595 has never really worked properly,.,.,the lower thread (bobbin) is very loose and then the thread breaks,,,I've tried to adjust the tension, but maybe I'm doing that wrong.

Start out with a brand new needle.

Be sure the machine is threaded properly. Also, the presser foot MUST be raised before threading so the thread will seat properly in the tension disk.

The standard setting for the upper tension is the midway point between the high and low number.

If the bobbin thread is still way too loose, you can try adjusting (tightening) the bobbin case tension. If that does not fix the problem, you can probably try purchasing a new bobbin case for your machine as the old one may be broken or worn.

adjust bobbin tension Google Search

loops under fabric means top tension needs to be tightened
try cleaning the top tension dial with some unwaxed dental floss and see if that helps. if it doesn't then the tension dial may need to be replaced
also check to make sure the spool of thread is not getting caught on the spool holder
also try a new needle--one that is correct for the fabric being sewed

There is loose thread hanging from the lower bobbin, isn't there?

Machine breaks needles
1.make sure you are using the right needle for your type of sewing machine.
Most machines can use generic Schmetz needles. There are cases where the machine will only use ones specified in the owners manual.
2.the presser foot may not be installed properly
3.use the correct needle for the type of fabric you are working with. To fine of needle on heavy fabric or to course a needle on silk or jersey can damage fabric. make sure the correct needle for silk is being used.
4.let the feed dogs guide the fabric & the feed dogs will work with the presser foot .
5.the needle is not all the way in -push all the way up against the stop

also clean under the needle plate & in the bobbin area for lint or loose
threads
check to make sure the feed dogs are completely in the up position

if none of these suggestions work then it's possible that the machine
is out of time & would have to retimed at a sewing machine shop

Besides the other advice here, check your bobbin case. At first I didn't get my thread hooked into the bobbin case properly and had problems that looked like the bobbin tension was not adjusted properly. I have to take the case out, put in the bobbin to make sure it's caught properly and then put it back into the machine. This is what worked for me.

Sounds more like the way the bobbin has been wound, is it loose and uneven or tight and smooth. Is it properly into the tension spring? being loose doesn't sound right, a thread under correct bobbin tension is usually pretty firm.

Is it loaded into the bobbin holder the right way ?

Also check the bobbin itself hasn't been damaged in some way, not worn/bent. It needs to turn smoothly in the bobbin holder so if one edge has been bent a little this will cause problems.

When winding bobbins, you must have tension on the thread so it winds smoothly with consistent tension on the thread. Most machines have a little tension disc on top or the thread goes through the regular tensioner in order to achieve this. But I always wind bobbins at about 3/4 speed and also add a little extra tension by holding my scissors handle on top of the thread spool so it doesnt bounce around. Also start winding with a little tail pulled through a hole on top of the bobbin and wind until this starting end is covered, then stop and snip off the tail. Then continue winding and watch the thread, giving it a nudge so that it fills evenly across the bobbin, not just all in the middle.

Think of it like the garden hose reel; if you wind it really fast the hose all goes into the middle of the reel, then jams up and you can't get the whole hose onto it. Same thing, wind smoothly and evenly across the whole depth of the bobbin so that the thread is smooth and even. Then when the thread feeds off it will be smooth evenly tensioned and should stitch out evenly.

There is some good trouble shooting information on this page too which might help you to work out what is causing this thread breakage.
http://sewing.about.com/od/sewingmachineindex/a/mtroubleshot.htm

check the needle to see if it's bent-roll on flat surface to see if it rolls correctly
thread tension maybe to tight-select lower number
sometimes the thread can get caught on the
1.spool pin
2.the notch on the thread spool itself
some times the thread will come off the thread take up lever

if upper thread is breaking
1. needle in backward
2.upper tension to tight-check manual for proper setting
3.needle maybe bent-try rolling needle on flat surface to see if it's bent & if so replace
4.thread may hang on bobbin case-check area for lent or fragments of old threads
5. knotty or cheap thread-always use good quality thread
if lower thread is breaking
1.the most probable cause of lower thread breaking is an inproperly wound bobbin
2.bent bobbin
3.lint built up in case
4. thread catching on a spring or latch

I would think the sew hook timing

try changing the needle, and make sure bobbin is in correctly