weired thing happened to me..the exact same thing..it was doing it for a few days and it annoyed the shit out of me..also being bad for the motor..then i realised that i blew a rad hose the day befor, and it wasn't running right since..reason..water in the spark plug wells..dosn't have to be much..or in everysingle one..a bit of compressed air, and unpluging all the the leads after the motor has come up to operating temp (hot) so the water can evoporate away..also wouldn't hurt to silicone spray your leads..