Skipped stitches and noises

Question

I just had the machine serviced and I sewed successfully with the walking foot while stitching in the ditch of a quilt. When I changed to a darning foot and dropped the feeddogs I could only sew a little ways when the machine would make metallic noises and skip stitches. The thread does not usually bread. I have changed the needle, changed the thread and changed the tension both upper and lower and nothing seems to work. Can you help? The technician is out of town for a week and I have important work to do. Geez!