RNMB

As you guys know, I just did my head gasket and the car was running fine in 3 days. This afternoon, I just developed a new problem. After driving about 1/2 hour, we stoped for about 2 mins and start the car again. The gear was in P, start the car, it rev all the way up pass 2k RPM and staty up there. Step on the gas to rev it higher then left the pedal, the RPM stayed up there and didn't come down. After turn off the car, pull off the key then start again several times, eventually it came back to normal. But the problem came back again several times tonight (not all the time).
Finally I found that when it happend, I shift the gear to either R, D, 3, or 2, then the RPM come down to normal position. If I shift to either P or N, then RPM came back up real high.
I affriad this is the tranny problem but one of my friend (none MB technial) think that it should be one of the sensor related to the throttle control. He thinks when I did the head gasket, I forgot to put back somethings. I don't think I forgot anything because we took note everything we did and also if I forgot somethings, than the problem should be there 3 day ago. Maybe some sensor just went bad today?
Anyone know this problem?
Thanks

RenaultFreak

there is a tiny possibility that the TPS ( Throttle position sensor ) is misaligned hence the ECU giving false readings...

w123_w126

FWIW I replace my idle control valve and control unit and my high idle went away.