Pfaff 7570 Tension Problem

Question

I've been trying to do some free motion quilting and my top thread keeps looping on the bottom side of the fabric (and I mean BIG loops). I've tried playing with the tension, threading and re-threading the machine, changing threads and just about anything else I can think of. This is one of those intermittent problems. I had the machine in for service for this exact issue a few months ago and they thought it was fixed. At this point I'm 'dead in the water' on all the projects I'm working on so any help would be greatly appreciated.