I have an old Singer (about 25-30 years old) that I still sew on occasionally and it does that also. I took it to a repair shop and they adjusted a little "thingie" on the bottom which helped but it still does it sometimes. The repairman said I'll probably have to buy a new foot pedal. Not too bad for a machine that's never had anything but cleanings done to it so I'm not complaining--just passing along info.