I have used 100 weight in the bobbin without any trouble and I don't always check before loading it. Pre-wound Nebbs bobbins are the best for the machine as a) there's more on the spool and b) they are all wound the same tension.
I assume that the trouble is with ALL the threads you've tried and not just one of them, so here's my checklist to try re: the bobbins if the trouble persists/ before making costly purchases. A. Slow the machine down to it's slowest. Sometimes the designs which are written for a domestic machine won't cope with the industrial strength. B. Check out the scratchplate for microscopic burrs. It only takes one needle break to make one and it can be really sharp. If you find one, file it with an emery board. C. Clean, clean, clean. Make sure there's no teensy little bits of thread that could be interfering with the top section of the bobbin case. It's often the simplest of things!
D. Look at the tension of previous pieces. If not even, then 1. load the bobbin into the case. Note exactly where the screw is. 2. Stitch out a small section of the design. 3. remove the hoop from the machine and check the back of the stitching. 4. If the tension looks either too tight or loose, turn that tensioning screw a TINY fraction. 5. Return the hoop to the machine and stitch out the exact same section you stitched before in a different position of the hoop. 6. Compare the two. If you've made it worse, turn the screw back to the original position and then the same amount you went the other way. 7. Stitch out the same section again in another portion of the hoop. Repeat 3 - 5 until the stitches LOOK okay no matter how many thread breaks along the way. THEN, slow down the machine to it's slowest. Watch every stitch and identify where the break is happening and whether it's on a particular needle or not.
If you are using old thread, it does get brittle and old could be as little as 6 months so give it the pull test to make sure it's still strong. Some times it's the digitising too. If the fabric is too thick it can cause problems too.
The very best bobbin thread I've used yet it the Brother thread. It just goes and goes without running out it seems! Invisifil (google) is an excellent bobbin thread too - but it is 100 wt. I've never had problems with it either.
http://www.not-just-threads.com/ also has a yahoo group with specials for group members, but is in Canada as is Invisifil.
I would really recommend trying the Brother thread which you must be able to get closer to home and if not successful, I'd say there was a bur somewhere on the scratchplate. Don't go buying large anythings until you're sure it's the thread for your machine though. I'm sorry I can't be more helpful - I'm in Australia and my suppliers would be too expensive. Let us know how you get on, won't you?