I had an old 830. One time, I heard this strange noise coming from my sewing room. I went in and found that the machine was racing on it's own without me being there.
I took it in to my dealer and he said there had been some problems with the original foot. He opened the foot up and showed me how he adjusted something in there. It was fine after that, but he did suggest I get a newer electronic foot pedal (a Bernina pedal). I did do that and it was expensive, but I never had a problem with either foot after that. Just recently, I took it to the dealer and tried to trade it in for some feet for my newer Bernina 440. But, the dealer said that there are so many old 830's coming in that he really had an oversupply of 830 parts and pressure feet. But, he did give me a good discount on the new pressure feet that I wanted. I'm thinking that you might check with some Bernina dealers to see if they have some extra 830 foot pedals around.