I have just finished the little quilt I had on my frame. I checked all of the things that you recommended except for the timing. I had already done a lot of work on it. I think my latest problem, however was caused by something very stupid. I believe that the skipping this time was caused by a cord that got between the sets of wheels on the bar. The machine was sticking on that. I know that was not my original problem but all of the other things that I did solved the problem until I put this quilt on.
I cannot thank you all enough for all of your advice. I am going to save it all in a file on my computer so that if I have problems with skipping again, I will know what to check. I hope I don't but this machine has been nothing if not touchy since I got it 5 years ago.