Mom3 said most of this while I was posting mine...My experience, and the training I've received, says that normally if the thread problem is visible on the bottom of the fabric, it's a problem with the top tension and if the thread problem shows on the top, it's a bottom tension problem. Most common correction is to simply rethread the top thread. It could be that there is lint in the tension discs which throws the tension off, the thread has slipped off the "track", etc. There's a dental floss that has a lot of texture in it...made for dentures, I think, and it provides some extra texture to help clean the discs, just run a length of it through the top thread track and run it back and forth a few times to clean the discs.