Try replacing the steering servo? It is known that Spektrum systems will glitch large when it doesn't get along with certain brand servo's. Then, even when you have a servo that works for a while and then starts glitchin, I believe (don't quote me one this) the tiny "filter circuit" (capacitors usually) part of the servo circuit board gives out and allows the interference to confuse the Rx, thus going into failsafe mode, turns, then shuts down.

Did the servo or Rx get wet at all? Did it start doing this after a bad crash or out of the blue?

That's all I can come up with for now...

Cheers.