I had a similar problem with my (original) Pfaff 1222 - for years....
I'd take it in and the tech would "clean" the foot control. And after a few times of using it, away she'd go again. I bought a used Janome and packed the old machine away because the tech told me it was a resistor in the "column" and way too expensive to fix and not work the time - needed to be shipped back to the factory and let their techs work on it.
Dug it out seven years later, it sewed fine so gave the machine to my DD. She used it without problems for a couple of years. When I was visiting her - and doing sewing for new baby - it was like seeing an old friend. But after sewing for what seemed like 8 hours a day for nearly a month - I love to sew - it started running on again. So I bought her another Pfaff 1222 off Kijiji - my DGD is now learning to sew on it (she was the new baby). Packed the old machine away again.
It was returned to me a few years ago and runs just fine as long as its not asked to sew for many hours at a time. Someday I'll switch out the foot control (bought a new one last year) and see if that makes a difference. But I have two other Pfaff 1222s so it's only used for teaching 4-Hers to sew....
I love that machine.