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