hello here from Denmark I just signed up for this nice forum my wife and I live in Copenhagen we have 2 U.S. Cars a Plymouth Fury convertibel 1968 and a 1968 Pontiac Firebird. so im not sure this is the right forum ,since this is a first gen pontiac but i hope is okey Pontiac we bought in July 2007 in Germany the same year was Imported from the USA Tennessee. When we bought the car there was a 350 TPI engine in. The engine swap is made in usa, and gives me a little trouble idling when I start the engine cold, it goes to 1200 rpm and stay there until I turn off it and start it again, then it falls to 700 rpm. if I switch to D Stay at 700 rpm, but I switch to neutral gear and back to D again it is back at 1200 rpm and I must turn the engine off and start it again to get it back to 700 rpm. is one who knows where I should look for the error

As I think about the system at cold temperatures there are only a few things that control the idle speed.
-- The knock sensor would retard the timing which in effect could reduce idle speed. Does not seem like your problem.
-- At Operating temperature an O2 sensor could technically effect the idle.
-- The IAC directly controls Idle by using a pintle that screws in and out to open or close a hole, this is controlled by the ECM.
-- The Throttle position, obviously controls the idle, there is an adjustment screw, to adjust it.
-- If you have a vacuum leak, this could cause a rough idle, or cause it to race slightly if it is small enough. To hunt down a vacuum leak we would listen to the engine for any hissing, we sprayed starting fluid while the car was running in the area of the hiss and if it changed the idle we considered it a problem...

I am not entirely sure, but if it was me I would try looking at the IAC or Idle Air Control. If you are standing looking at your engine it would be on your Left side (right side of the car) of the Throttle body. There are 4 wires going into it. I might attempt to clean it and see if that fixes it as it sounds like it may be sticky. It is a wild guess but a place to start...

You may try to set everything back to stock, check your throttle position sensor, the minimum idle, etc then go from there. Chiltons or Haynes manuals should lead you through all of this to set it.

There are probably several other things to check, but that is a start.

John