Overlock thread! There is your culprit, methinks. Overlock thread is not intended to be strong. It is used with several other threads to create the same seam. It is not intended for quilting. I would personally not use it for anything except in a serger.
What you need is a good quilting thread. Yes, you can use 50/3 which is 50 weight and three ply, or strands twisted together. But a better thread would be a 40/3 with the appropriate needle. Try a polyester thread, but not spun polyester. All threads are not created equal! Some are much more equal than others, however. Get a good quality thread and let us know if that solves the problem.
If it does not, then let me tell you that I have tested more than 100 machines for free-motion quilting, and out of those 100 plus, there were maybe 5 that did good fmq and another 5 that did reasonable fmq. Another thing to consider is the individual machine. I used one Janome 7700 and the fmq was great. I used another one later and there was endless trouble (and yes, the thread snapped and I did all the various tests of thread, etc., that you did). In both cases, the dealer was there and for the second machine, could not fix the problem.
In many cases, machines with vertical bobbins do better than machines with horizontal bobbins. One exception I know about is Janome. I have one of the smaller Janomes and it has a horizontal bobbin, but I have never had it jam with fmq or thread painting, or anything else, for that matter.
For the future, remember that quality of thread and type of thread makes a major difference.
Last edited by cricket_iscute; 12-29-2013 at 02:33 PM.