i have an lt1 in my datsun, and a couple of weeks ago, i started having this problem with the car. when it would start up, it would idle really rough for a couple of seconds, so i would hold the rpm high, then it would go back to normal. then, it would idle high sometimes, and low sometimes. now, i went to start it today, and the battery was dead(i do not know why, because i rewired the entire car with a painless harness. I have the vats module with the little red light on the top of it, and when i turn the car off, the red light stays on. Could that be whats killing the battery?) so i put another battery in it, and now it will crank but will not start. I was going to see if there was spark but it was raining. The fuel pump was kicking on too.

 

Any codes?
Check the fuel pressure, even if you hear the pump priming, doesn't mean it's actually working.
Check for spark at the coil and at the plugs.