I'm not sure why the thread is acting like that but your quilt is just gorgeous! I hope loosening the tension on the frame helps!

I had this issue with a quilt top that just started coming apart for what seemed like no apparent reason. Well .. come to find out my iron was over heating and the thread became brittle. I had used a poly cotton for the piecing. I had not realized what was happening withthe iron until the problem became extreme and I saw the thread literally disappear before my eyes as I was pressing a block on the next project. Then I knew what was happening with the quilt I had just completed that different parts just seemed to have the thread vanish.
Needless to say that iron was in the trash so fast.

No...it's a spool I picked up about 6 months ago. 100% cotton Aurifil.

Your Alphabet, Math Quilt is Adorable !!
OMG you may get to keep it for yourself, :) then can mend when snuggled up in it this winter, eventually you will have the whole thing done by hand. Easy Peasy ;)
(just teasing) Good luck with it.

Only thought to why or idea why it's doing that, have ran across dropped stitches at higher speed, and they stopped at lower speeds may be time for a service.

Your piecing looks gorgeous. If the tension is tighter on one thread than another, I could see how it could snap. If that were the problem, though, I would have thought you'd have caught it when you were pressing.
Skipped stitches sound more likely, but again, you'd see that in the pressing.
Strange. Must be the quilt gremlins.

I have a Bailey and I find some times I tend to push my machine faster than it is build to do. And when I do the stitches do what you are saying yours did.

Your thread could be too old, I had this problem with some of the thread I had for years, I threw it out.

Probably it is either aged rotted thread or poor quality thread that is just not holding up to the stress of being in the quilt. Old thread is weak thread. It will rot from sun and time just like any fabric will. Also I do not use Serger thread to piece with as it is just not as strong as regular thread if used singly, it is meant to be used in concert with two or three other threads in a totally different type of seaming. You don't have to spend a lot of money on thread though, the standard brands like Coats and Clark, Gutterman, etc. the kind you find in chain stores is good strong thread. If you sew a great deal and want to really save then go to thread on cones. I buy mine from Newark dressmaker supply, they have a web store. I get some cream, medium gray, and a dark, and that will do for most piecing, But I do buy colors too for machine quilting. Here is how to test your thread, take about an 18 length and get a firm hold on each end. Give it a sharp tug, sort of quick pop, if it breaks it is rotten. Now pull strongly on it, good thread should stretch a little but hold, cheap thread will not. I also invest in Bottom Line thread for the bobbin. It is more spendy but.....it is very fine and will go twice as long in a bobbin than regular thread, it is super strong, you must cut as it will not ordinarily break, the cones are very cost efficient. I buy a light Grey, almost a silver and use it for machine quilting. It just disappears into the back with very little pokies. Remember you are the most important tool in your sewing room and you labor is worth more than any other of the supplies you have. You deserve to have the best fabric and thread you can afford. Cheap thread is no bargain when your wonderful creations fall apart too soon.

the quilt is beautiful. And I love the quilt frame. I would like to have room for a full size frame.
I have made machine pieced tops and there are holes in the stitching. not really holes but gaps. I have to go over it after it is quilted and washed and do a major repair job. no fun.
but have found with a hand pieced top no gaps. maybe I am more careful or maybe the thread is better not sure. But I like the quality of hand pieced and hand quilted.

Sounds to me like a tension problem. You are using good thread, and I assume that you didn't notice skipped stitches??
We all know that bobbin and top tension needs to be balanced, and when it is the interlock of the stitch is centered in the fabric sandwich. However, you can have what looks like a great stitch and still have tension problems.

Are your gaps where you have a cross grain sewn to a lengthwise grain piece? Probably are. Cross grain has some give/stretch, lengthwise doesn't. So if tension is put on the seam the thread can break. (and you really want the thread to break as opposed to the seam line ripping!!!)<G>

It may be your bobbin tension is not holding well. I recently had that problem. The spring that holds the bobbin in the bobbin case had worn. So I had some real problems with my stitching. Things like bag handles, which I make a lot of, were suddenly having popped seams. Forget trying to get a good stitch on bias edges. and so forth.

Once I had the bobbin case fixed, the problems went away. <VBG>

Hope you find the culprit,
Pati, in Phx