When I took a beginning sewing class I learned about thread and "slubs". Some thread is very "slubby", if you hold it up to the light you will see a lot of nubs along the length of it. This is what shreds and gums up the works. I went home and looked it up on the internet and found a site that had different threads under a magnifying glass and C&C was one of the worst offenders. Gutterman and Mettler were the best.
Since then I have heard Coats and Clark have improved, and I bought some 100% cotton for my last two projects. The thread broke more often and the machine had a lot of lint to clean out, but it did work.