With the robot is on the it's cradle and powered up, by operating just the forward trim on the Tx do both sides move very slowly at the same setting? If not then swap the speed controllers over and try again, if the symptom changes the problem is with the speed controllers or the controls Tx Rx or wires, if the symptom stays on the same side it is the motors or transmission.

Are the wheels free to move are the motors stiff? If the robot has been in combat things get bent and this can cause problems.

Some motors are configured to work better in one direction than another, this is done by magnet position I don't think this is a problem for you at this time.

dose the problem only seem to occur when the motors are under load? so on the cradle it's ok but on the ground it causes problems. If so it is down to the power being delivered to the motors this could be the speedo is bust or the circuit resistance is for some reason too much.

The wire and plug connections can be a problem Tamiya plugs can cause problems I am getting rid of these on my robots also check for damaged insulation on wires also check soldered joints and avoid using screw connectors (choc blocks) as they can cause problems.

Keep trying different things and soon you can narrow the cause of the problem down.