The other thing I may check is to see if there are any broken connections to the capacitors, as they seem prone to breakage on the 9920. Obviously your problem is tied to heavy loads on the controller, and there only so many things that would cause that.
Edit: Had another thought... sometimes I got cogging when running a ubec even w/ an FM radio. The DSM radios are the only ones that ensure smooth operation. Have you tried using a RX pack in place just to check this out? What radio
thanks all! i think you are right about the batteries. I recharged the batts and ran the truck with batts hot off the press. cogging almost went away. however, things got progressively worse as batts drained. looks like i may need lipo, but a little too much $$ for me to spend now. i'll try and gear down to see if it helps.
I had a very similar setup and I think that gearing down will help alot, maybe a 13 or 14t pinion would be better. Those batteries may have some issues moving the truck around properly with taller gearing.