Bulldogger Ok so you have a problem. Find the issue before you going doing stuff or you may find that the issue will continue. Look for shorts ( metal tiger ) then as you said is the drive train running smoothly? are any of the pins coming out of the track as this has been known to cause driveabilty issues. The power draw on the fets would be high and if you did any turns BUT here is the issue - the metal tiger comes with the Mato's 3 to 1 gears. Their should be plenty of torque from those. Are the motors good?
All in the cost of an RX18 is pretty low but then all you get is a toy controller.
Take a hard look at everything before you move forward though just to be safe.
More than likely it is the board but I can't see the tank or what it is doing so take my advice with a grain of salt