engine runs rough plus check engine light is on

I have replaced plugs, wires, exhaust pipe, o2 sensors, all by a mechanic......  he says it's probably the injectors.  It runs well on cold start, but if i turn engine off and restart it will run rough the rest of the time.  I will gladly put more money into it, but I'm tired of dumping it into a black hole.  Please Help,   Arch