Omni-wheel accuracy

[mrcheese123]

I am currently using Robot C to program with a tetrix kit. We have a base drive train with omniwheels and is setup like so:
/------\
| |
| |
\------/

I already have the code for telop written but in testing the motors have a big problem with rolling. When the bot goes diagonally, the motors slow to a stop at random (not every time) rather than just stopping. Also, occasionally, when the train is rotated, two motors stop and two motors roll to make it roll about an extra 45 degrees. Note that this occurs with different motors every time. Does anyone have any ideas on how to compensate for this?

Message me if you want to see the code, I don't want it leaking out everywhere.
Note: this is an FTC robot.