I found the sabertooth speedcontrollers to be pretty good actually in that respect. Have you left the speed controller in auto calibrate mode? We found a problem that if you get abit of a voltage drop and the ESC re-boots then it messes itself up by calibrating with the centre stick position wherever you joystick may be at the time.

Other problems could just be that its getting stuck and caught on the arena floor all the time and making it go all over the place.