I got the RSX, installed it, and took the truck for a run. Turns out it was the ESC after all.

Now I have about double the power I was getting before. I didn't have the slipper cranked down tight enough, so I wasn't getting quite all the power the motor is capable of.

I gotta say I'm impressed with that little truck. it isn't as stable as a 1/8, for obvious reasons, but it is pretty quick..and can get nearly as much air as the buggy. it's way quicker than my old dest210 w/ 13.5t.