PaperPrincess gave great suggestions. If you have a bobbin case, try tightening the tension 1/4 turn at a time. Clean & your bobbin compartment with a small brush & oil where indicated. When I can't find my little brush sometimes I just use a long pin that I dampen with a wet paper towel to grab the dust bunnies & then wipe the pin on in each time it grabs some dust. If you have a bobbin case, check for any little burrs that could be shredding or cutting the bobbin thread -- which would then create a thread nest (same thing that happens if you don't pull up your bobbin thread to the top of your fabric before you start sewing).
And, of course, it's always possible that it's the thread itself that's causing the problems. Try a newer or better quality thread & see if it still happens. If you're on a budget, I'd go with either a 100% poly thread or Gutermann cotton thread.