Motors are rotating at different speeds

We are competing in Vex Sack Attack… We have our robot built but are having some problems. We have 2 wire 393 motors on each back wheel and the omni wheels up front. For some reason the left rear motor keeps cutting out and not the back right motor. It seems like the left motor is going slower than the right. Why would that be and if it is could that be why it is cutting out and not the right motor? I check the programming, they are the same set up on each wheel… We use Easy C… Can anyone help or give suggestions? We run the robot for about 1min 30 sec and let it cool for about 1min 30 sec… It will go for several “rounds” but after that the left motor keeps cutting in and out. I can understand if we are over heating them, but not both motors are cutting in and out…

Thank you,
eloveless

Have you tried using a different motor?

Are the motors on each side chained or geared together or are they separate?

Replacing the motor should fix the problem. Vex motors don’t last forever and this sounds like pretty typical behaviour for a motor that is near the end of its life. It might also be worth checking whether plugging the motors into different ports has an effect.

Have you checked the friction on the two sides? A wheel turning slowly and then the motor cutting out is one typical symptom for a drivetrain with too much friction.

Kinda simple but have you changed the internal gearing on any of the motors(put into hs mode)?