I no longer use coats and clark thread due to a similar problem. I try to use Gutterman, aurifil or superior thread. Although I have a lot of metzler thread I am using for piecing too. But as it is used up, I will switch to superior or aurifil.
I also only use 100 percent cotton for my piecing.

Are most of the thread breaks on bias seams? If so, sounds like the bias is stretching a bit and breaking thread. I've had this happen while loaded on the longarm. I have loosened the quilt while on the frame and no longer get this problem. Hope this helps.

You can use the blind stitch that you use on the back of the binding rather than the whipstitch and then it wouldn't show so much. At least that's what I'm thinking.

The thread this was pieced with was 100% cotton Aurifil...so I don't think it's the thread...if it was the thread it would show up in more placed than just in the same area in the blocks where it keeps showing up at.

The seam is not a bias seam, but I will try loosening the tension on the quilt in the frame. It is always a seam that runs horizontal to the bars of the frame, so I might have too much tension on it. This is the first time I've used this frame.

Hope it all works out! The quilt is very pretty!

I made a pair of lounge pants for my son and the same thing happened to me. So strange!! Can't figure it out.

Good quality thread pays for itself. So far I haven't found any problems with Connecting Threads Essential thread. I sewed all my small projects and table runner with it before using for piecing quilt blocks and was satisfied with the quality.I buy all the good brands when I find them on sale. I have YLI, Superior, Aurifil, FilTec, Robin Anston, Metteler, Sulky, Guttermann, and InvisiFil. If it's on sale and a quality brand I buy it.

Wow, sure sounds like the thread...

are you using old thread?