Have a look at the lock first. Mine has just started to bind up due to wear. If I push the lock down just a little the peddle frees up. I just need to adjust it. Adam.
The drive motor may need a seal kit. I seems to be something that fixes jerky movement. Mine is doing something similar but not to the extent yours is and I have a kit that I'm going to change it next week so I'll let you know how it goes. Cheers. Adam.
My 751 was doing the same thing. It turned out the case drain hose from the control valve to the tank had been chaffing and had a small hole. I found it after giving the machine a good clean. Adam.