this happens a lot but not all the time but when i start my 00 gt up from a cold start and shift either into reverse or drive, the rpms will drop below 500 and then shoot up to 1,000 and jolt the car slightly, but after that its fine. whats goin on here?

 

Mine does this also. The RPM's fluctuate while I'm sitting stationary in 1st or Neutral and will occasionally drop below 500 but normally vary between 600 and 900. My dad has a manual diesel truck and he said that it is normal for the RPM's to fluctuate like that but I think it's a bit extreme.. any thoughts?

 

IAC Valve maybe.....dirty throttle body.....clogged air filter.....any trouble codes?