Sounds like it needs a service. You could try cleaning it thoroughly with compressed air/ little brush including the bobbin holder, you may find threads tangled up inside, and then oiling it, but if no joy take it to a professional.
I have an old but very serviceable Pfaff sewing machine, however, the foot won't go down properly and actually when I press the lever the foot goes down in slow motion but doesn't then grip the fabric. Anyone got an suggestions on how to fix it? I don't want to get rid of it because it is otherwise a good, solid machine.