Isolate everything... by that I mean, systematically isolate what your problem area is. Install working servos to the reciever, use different a radio system you know is in perfect working order, etc. If you don;t have extra radio gear, nor a friend with extra radio stuff, take it to the hobby shop... they should be able to do this for you as well. I mean you could have a bad battery contact plate in the battery plug, a fried reciever, broken crystal, horrible glitching- among many other things.
But most likely, this is something to do with your reciever. Like Tabushi asked: did fuel, water or just a load a of crap get inside your reciever ever?