So it cranks over just fine, but a few seconds after the initial rev upon startup, the revs drop quickly and it shudders and dies, or if you put it into reverse it dies. start it again a second time right away, and it almost always keeps running. Sometimes if I rev the motor immediately after it starts it won't die

If the engine is really warm it may not die on the first startup. what's up with this? Idle motor has been replaced, so that's not the problem. Help is appreciated!!! our mechanic can't figure this out!