high idle on '97 Tbird

Hi,
lately my car has high rpm's when in D (auto gear box offcourse) when i'm standing still (4.6L). The rpm's vary between 900-1100 which is to high,
then after a while, say 30-60 seconds, the rpm suddenly drops to the normal level of around 6-700 rpm and then after another short while they drop to 400 and sometimes this causes my engine to stall....
During slow driving this is even more anoying (below 20mph), cause then the rpm is around 12-1300.
What could this be??
Thanks in advance,
Michael
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Clean or replace IAC valve.
Michael wrote:

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