And if Pat didn't give ya enough to check (good list Pat):
Bad or Loose Crystal
Bad or Loose Solder Joint
Receiver and/or ESC that has been ~bumped~ to much (I.E. one hard crash will do it)
Low Transmitter Batteries
ESC servo/receiver wires to close to Motor, Battery or Battery Wires, or frayed insulation
Dirty motor / comm / brushes
.... I could go on, but that should be enough to check for awhile
