300zx having trouble starting?

have a 94 300zx and it has been acting very weird the past few weeks. It will start up just fine after it has been sitting up over night or for several hours, but if i kill the motor then try to crank it up it turns over but will not crank. WHat could be the cause of this, when the car starts it drives with zero problems it just will not start once it has been killed for at least 2 hours. the dealership where it is at has had the thing off and on for 3 weeks and has come up with nothing. they took my aftermarket alarm off, checked for carbon build up on the valves, looked into the EGR valve but it was fine, cleaned my injecters, checked fuel delivery system, starter and nothign is raising any flags. I was told it my be a starter realy switch because they often check out to be fine on diagnostic test but are actually bad. ANy input or suggestions would be great