Sounds like your trying all the right things. Two more to check is 1) the tension in the bobbin case -try pulling the thread through the case if it comes out fast its too loose if it is hard to pull out its too tight. My Pfaff book says you should be to hold thread and drop case (like a yo-yo)and it should unwind a little. 2) check to make sure the spot on the bobbin case where the thread goes through is not bent, I bent one years ago and it caused all sorts of issues (thread nests, skipped stitches) until I figured it out and "bent" it back towards the bobbin.
I probably wouldn't spend $85 to have someone look at it but put my $85 towards a newer machine from a sewing store (LQS, JoAnn's, Hancocks, etc. and would make them let me test it first