gaz@?

were do i start ??? some days its ok, but wat i have found is that if i leave it ticking over after a run, next time i try & start it the check engine light comes on, the engine will crank 4 ever but it wont fire.
if u give it a bump, & start it with key again it will start no problem, if u switch off the engine b4 u stop it starts fine the next time.
i have just put new plugs in & didnt make any difference
plz plz plz help its really pi~~ing me off now.