this happened to a friend of mine(on a 1/8 nitro),the car kept going into failsafe after only a few turns of the servo,having changed batteries and tried a different receiver it was still the same
after faffing for ages it turned out the throttle and brake servo(savox 1258) had a very small nick in the earth wire.he put a different servo in and,problem solved!
May be worth checking your wiring mate.