Dying Drive

Our drive is completely normal when we try to move forwards and backwards, but when we turn, the front two motors turn really slowly and die it in about 10 seconds.
We use a direct 4 motor highspeed drive on 4" omni-wheels.
Here is our code for our arcade drivem, the left two motors are y cabled and the right two are y cabled as well:

motor[LeftD] = (vexRT[Ch3] - vexRT[Ch1]);
motor[RightD] = (vexRT[Ch3] + vexRT[Ch1]);

Thanks in advance!