Sounds like interference or brown-outs. Try the new speed controller first.
To me, it act's like an FM reciever when it loses it's signal. That is Exactly why i switched to spectrum technology.
I crashed my Trex 2 or 3 times and it was horrible. It would just get a mind of it's own and power plant itself right into the ground.
Wireless Routers and Cordless phones do affect fm recievers. I have not had anymore "what the heck just happened" incidents since the switch to spectrum.