this past month my truck has been acting up. about 2 or 3 times so far, when i go to start my truck, the engine fires and immediatly dies, without idling whatsoever. the only way it will start is with pressure on the throttle. i gotta hold it there for a few seconds after start up or it will die tho. and once it is idling, its too low (500rpm) for a cold idle. any ideas? the ecm was just replaced two months ago with a "rebuilt" one.