Personally, I believe it's the receiver pack. The radio system needs 6 volts to work. 4 alkaline batteries will do that (1.5 volts per cell). If you run 4 rechargeable cells, they are only 1.2 volts per cell so you only get 4.8 volts and the fail safe detects the low voltage. I would suggest trying 4 alkaline batts or go to a 5 cell rechargeable receiver pack.
I had that same problem with my first nitro car.
Go Fast, Turn Left, Repeat as needed!!
It's not as easy as it looks!