Well, since it won't sew more than one piece of fabric, I'd start by making positive sure you have the correct needle in the machine.
It takes a regular 15x1, and that the needle is in the machine correctly. It goes in Flat to the Back. Then make sure that it's threaded properly, the thread goes in Front to Back.
Of course this is after making sure the bobbin is good and the bobbin area is clean.
Since there was trouble before, with the timing belt it might be advisable to double check the hook timing as well.
Joe