[BBray_T1296]

I think, more precisely, you are setting a 'power' value, less directly linked to speed than you need, which is the root of the problem. a motor will operate differently if it is trying to oppose your forward movement than add to it.

If you measured an unloaded CIM running at "50%" PWM, you will find it attains a RPM far greater than 1/2 of its RPM at 100% PWM, but with a loaded shaft, you can make a motor stall at 50% while it can still run at 100%.
Consequently, a robot moving forward and suddenly sliding left, will operate funny as several motors try to reverse direction and cant keep up (because of the opposing force [mainly inertia/momentum]) with motors continuing to spin forward (with far less opposing force)
This has a large effect, in addition to the above stated "motor preference"
Unfortunately I know next to nothing about software (a certain teammate of mine will attest to that), but I hope this makes sense as far as the problem is concerned.