734D Superlock --- always very reliable, but now it's skipping stitches. I had it in for repairs, but the tune-up didn't last. I have put two brand new needles in it, and followed the threading directions to the letter. Also tried adjusting the tensions. I am tryng this out on cotton fabric, well-loaded with sizing. Nothing seems to help. What is wrong?

Sewing Machines - White Sewing - Speedylock 1600 Mechanical Sewing Machine