Here's a possibility. I had my machine in for a service and my guy pointed out I had damaged the throat. Broken needle perhaps, but it left a burr that was breaking some thread and not others.
He polished it out and I have had no trouble.
MaryKatherine

I have a Babylock that doesn't seem to like FMQ. My daughter has been doing FMQ for about 4 years and she couldn't get it to cooperate, but we are not done with it. Next time she comes we will try again using different threads, needles, etc. I have a Pfaff that does nice FMQ so will use that until we can tame that Babylock tiger!

I have a Janome 9700. It doesn't FMQ right with the feed dogs down. I use a SUpreme Slider to cover the feed dogs and it solved the majority of my problems. I had thread breakage problems with the feed dogs down and also eyelash problems in the bobbin tension. Depending on the thread I sometimes have to mess with the needle size too. With the feed dogs up the only problems I have are operator error problems. Also check for burrs like someone said earlier. I had that problem too. Definately try different brands of thread too. My old machine refused to sew with Gueterman thread.

I use size 18 embroidery needle longer eye. Size of threat also counts. Sometimes lowering the top tension helps. Old threat also breaks easier. hope this helps

Leah Day has said that her machine doesn't like certain directions (like back to front or right to left), but you should be able to go at least a little ways. My machine prefers polyester thread.

Research thread. You will find that serger thread is considered "inferior" for sewing machine use....it is a weak thread that will fray and break. It is designed for use in overlocking, where numerous stiches locking in numerous areas, will keep serged fabric from separating.....when you use it in a single stitch fashion, it is one lock of the bobbin and upper thread. It causes extreme lint in tension disks and machine parts. This same lint builds up in the eye of the needle as well. My first reaction to your comment, was OMG...serger thread?....nine times out of ten if you have thread breaking while FMQ, IT IS THE NEEDLE SIZE AND MATCH TO YOUR THREAD. Ask me how I know..took a FMQ class with my new Janome 8900. ( I had no problems with the machine FMQing right out of the box) a few days later I was taking the class....broken thread, broken titanium needle..boy do those things zing across the room!,,,...the instructor checked my needle and thread.....it looked fine...it was the 90/14 she recommended for the class. I happened to turn my cone thread upside down, in the end was a label printed with " use 90/14 microtex needle with this thread. Wow! luckily I was taking the class in a machine dealer store, bought the right needle, Immediately, great FMQ. I became a believer in matching the needle to the thread and use..... change your thread to a better quality, 90/14 topstitch or microtex needle and you should be on your way.

I found that if I was having problem with the thread breaking, I gave up and upgraded to more expensive thread and not one breakage where before I was having a problem once a minute or so. Same everything, except thread.

When I found my first choice of thread didn't work, I tried using the embroidery thread--expensive I know---Isacord--but because I bought a "lot" on Ebay, it wasn't so expensive.

But I also noticed on my Horizon that if I had the needle hole open (as if using zigzag) that the thread broke more often. It worked better with the hole closed as if for straight stitch, if you have that option on your machine.

I know that there was a discussion on here a while ago about using serger thread for piecing, so I didn't chime in, but honestly...I use what I have.

At the wholesale place here in Poland, they don't tell me, "It's serger" thread--they tell me how strong it is and what it is made out of. At that place they sell all kinds of thread from elastics to Gutermann, to thin -as-cheap-as-it-comes thread. I buy a middle of the road thread for most of my sewing, actually.

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.

I noticed in post #18 you mentioned 'overlock thread'. You should never use overlock thread to quilt with because it has very low tensile strength and will break easily. Its purpose is to be used in a serger and you are inviting problems when you use it in a sewing machine. There is a reason that it costs much less than spooled threads of various brands. The information presented by Dr. Bob of Superior Threads is very well done and very informative for all experience levels of sewers. I'm not sure which quilting aids are beneficial because we all have different ways of holding things, etc. I used to use the plastic washers, the slider, machine gloves, etc. However, after lots of practice I do not use any of them and am pleased with my FMQ. It is now my favorite part of the quilting process.