[Alan Anderson]

A six-wheel drive base will tend to go in whatever direction the frame wants it to when the motors are driven with approximately the same power. I'd definitely look for warping or binding somewhere.

However, I'd also double-check the software to make sure the speed controllers being defined in the code match the ones physically installed. If you're using Talons, you must select Talon from the menu at the bottom of the Open 2 Motor Drive function. If you use (e.g.) Victors while telling the software you're using Jaguars, you'll get asymmetry in motor speeds.