When the car is warmed up, if I rev the engine in either park or neutral, the rpm goes up and then down and stay around 1500 rmp and will not go down below 1500 rpm. If I shift to drive or reverse, the rpm will be back to normal back down to 700-800 range.

I don't think this is normal. I did just passed the CA smog with no problem recently. Anybody got any ideas?