Thread tension for heavier fabrics using heavier thread is a tricky business. Here are a few things to try. Definately adjust the bobbin tension. If it is too loose, it could cause the bobbin to spin freely to cause the rat nest. You do want the bobbin to fall a few inches after a slight jerk of the wrist, when suspended from the thread. You should also try running the upper tension higher than normal; perhaps just above what the machine says is the normal tension range. Sharp needles can cure more problems than you might think, and they dull quickly or bend slightly when sewing heavier fabrics. A final suggestion would be to try putting the spool of thread on the ground, and running the thread over something so it feed correctly back into the upper tension as normal. The idea here is to have thread come off the end of the spool rather than unwinding from the spool. Some high end machines are designed to work this way normally. BTW, these tips came from my local sewing shop.
Some of the parts around the bobbin can get damaged easily by a misguided needle. This creates burrs that catch the thread and do some strange things down in the bobbin area. If the above tips don't help, I suggest seeking professional help...um, I mean for the sewing machine. :) Good luck.