So I had to get my sewing machine serviced. It's the first time I've had to have it cleaned, and it was about time. When I picked it up, the repair guy told me my thread was crap. He said Coats & Clark was ordinarily a decent company, but apparently they've taken to selling lower quality stuff to the chain stores. That was his guess, anyway. Maybe they're just no longer a good company. It was marked as machine quilting thread, but it wasn't a consistent thickness, meaning that it was causing my poor sewing machine unnecessary stress and breaking more often than it should. Whew - it wasn't my technique.
Coincidentally, we had a guest lecturer give super huge high praise to a thread I hadn't tried - Aurofil Mako Ne. I picked a couple spools at the local quilt shop, and I'll give it a try. It's pricier than what I was using, for sure, but if it pieces and quilts as awesomely as they claim it does, it will be money well spent.