This may be too late to be of help, or not fix your problem, but I am having the same problem. Or rather, was, until about 10 minutes ago. I was given (!) an 830 Record Electronic last weekend, and it was also only sewing at top speed, only at the very end of pressing the foot control. I had assumed it was the capicater, based on googling. I’m trying to do all the refurbish/maintenance on this myself, especially since there isn’t a Bernina shop all that close. Saw that there is an adjustment screw on the bottom of the foot control, just a small screw with a + and -. I turned it towards the - side and it stopped running completely, turned it all the way to the +, am due it is suddenly working, at all speeds. The motor is sounding a little clunky at super low speed, though. That’s how I found this post, I’m trying to figure out how to take the end cover off to get in and oil that side of the machine. It sat unused for about 20 years, uncovered except for being lowered down in a drop down sewing cabinet, in a smokers house.