Backpack sized off-roader?

Had it out today in the snow, and it suddenly started cogging, really badly. To the point where you almost need to stay on full throttle to avoid it. Batteries are fine (Using the standard packs), sometimes the steering servo -when turned slowly and not moving the truck- doesn't respond until you give a bit of throttle, even a single 'cog' to send some power through the truck. I tried changing the CH1 position on the reciever and the problem remains. I also re-linked the Tx/Rx, still the same problem.

Here is a video of what happens when I put two 6 cell packs in series and try to run the truck:

http://www.youtube.com/watch?v=Xt0y8Cx8lQM

Any thoughts?