Vectra starting problems?

Hi I have a 1999 1.8 ehco tech Vectra auto which has had the head gaskets and rings replaced because of a very bad over heat. The head looks ok and the engine has now been rebuilt. It takes a very long time to start you have to keep turn the engine over and after some time it will start. no smoke from engine. When the car is running and driving its fine picks up ok. as soon as you stop and turn the engine off it will not start if you leave it to get cold it still will not start. can someone help ???????