What the temp of engine at normal running.
115 deg C
If that alright you just have to do some maintenance on the clutch you have.
The clutch is good. the motor is not revving up. The moment it revs up, the car goes of quickly.
Slowing down the speed of the throttle may help. and propapbly rectify half the trouble you have.
The problem is only after a couple of minutes into the race, so I dont think its the servo being to fast.