How can I get my Brother Pacesetter xl703 to not tangle under fabric?

I got this machine used without a manual (I would like to find a manual for it). I am not sure the bobbin it came with is the correct one for it (again, a manual would be useful here). It stitched fine for a while, but now it is tangling and leaving large loops on the underside of the fabric. I was hoping it was as simple as a stray thread so I did a minimal cleaning/oiling but the problem persisted. Results from adjusting the upper tension knob have yielded a really nasty tangle right below the dog teeth and now I am fearful of damaging it by experimenting further without guidance. Budget will not allow for a repair shop at this time.

Help! :)