What brand of invisible thread are you using? I had trouble with Coats & Clark after they went to the new pink spools, the thread will wind itself around the thread spindle, and tighten and break. I have switched to YLI, on a spool shaped like a little silo, a little more expensive, but has 1500 yds on the "silo", and doesn't have that problem at all. If you have a lot of C&C spools that cause problems, I found a product called Baby net, that is netting to use on cones of thread. They are long, so cut one in thirds, and slip over the thread spool, and it will stop that annoying breakage.
Sie