It may be defective motors? Have you an ohmmeter? Disconnect the motor at the MFU, and measure resistance from motor terminal to the motor case, on both motors. You should get infinity or high resistance, in the Meg Ohms. Check than none of the capacitor wires on the motors are shorted to the case, or touching anywhere they shouldn't be. I believe normal construction is to have 3 per motor, one between the motor terminals, and one each from case to motor terminal.
Be careful with this, since its easy to fry an MFU if it is drawing too much current. Is the MFU getting hot?? Are any of the power leads getting warm? The stock wiring is rather marginal, so increasing to the 390 motors can bring on issues on a heavy tank like the KT. Have you done any modifications to the electrics? What battery are you using?
Jerry