Gsr Problem with Idle

I have a 2001 gsr and whenever i rev the engine to 3000 rpm-in idle and let go of the gas, it goes all the way back down to like 200 rmp then adjusts itself to like 600-700 rpm. Is this normal, it happens more often in hot weather, could it be the head gasket?

My B16 does that too. I have adjusted the idle a little and it seemed to help. I just live with it. I hasn't effected the performance or anything..

Check the IAC valve