My clocks seem to do this all the time. They will work fine for days then stop for 1or2 hrs then start working again.
My gearbox has just gone - so could have had something to do with that. I believe the sensor is on the gear box - but not 100% sure on that one
just check the plugs on the engine and make sure everything looks plugged in, etc - dont think its anything to do with behind the clocks