If the servo does not act up when it is outside of the vehicle, that points to a couple of things. Either it is heat related from an outside source (engine) or the more likely cause would be from the end points or linkage not being set properly which is putting more of a load on the servo and causing it to act up once it causes the circuit to heat up. Thrpple check those end points and the linkage. Also are you using an ABS brake function in the radio?