I agree with Prism. I'd also try changing the thread -- top & bobbin. And if you're using 2 different threads, try a test piece using the same thread (or at a minimum, the same weight & fiber content of thread).
Another thing I'd try, although I doubt it would cause loops, is making sure to give everything a good cleaning. Dust out your bobbin case & "floss" your machine by running the thread through to just before the needle & then pulling it out from by the needle. Sometimes the tiniest fuzzies can completely ruin a project.