How can I fix the zig zag on old Morse Fotomatic III?

All of a sudden my machine doesn't want to zig zag. It seems to work okay when I manually turn the balance wheel but when I use the foot pedal the zig zag is almost non-existant. It almost seems like the needle can't move left to right fast enough. I am also getting an occasional thread jam in the bobbin. I have tried changing the tension on the top thread and on bobbin case but it didn't fix the problem. Any suggestions would be appreciated. Thanks