Hard to say, but generally the potentiometer (pot) in the both the steering and throttle will tend to wear out over time... some brands of radios have replacement parts so you can swap out the trigger/steering if necessary, some brands will let you ship the radio to them for service as well.
If the radio is pretty old, or the electronics have been abused with excessive moisture, then I imagine that the radio has been damaged.
Sometimes you can get away with re-calibrating the ESC, but no guarantee if the pot will hold the settings anymore.
Good Luck!