I have and love to sew on a 830R. Have you checked the max/min slider on the front of the machine? It does control the speed at which the machine sews. It is on the bottom front of the machine, could it have been moved from min to max inadvertently? I usually sew on the min setting as the max is way to fast for me. The foot pedal is the only part I have ever had to replace {twice}. If the pedal is not working right it will not let you control the speed with the slider. I had the part replaced by a dealer but my understanding is that Bernina no longer produces parts for the 830R as the factory had a flood. Could your dealer have replaced with a part that is in reality generic....just things to check out.