recurring problem

Now that the weather is warming up in NYC I am getting this recurring problem with my 1990 Grand Marquis 5.0 EFI.
When I come to a stop and change gears, like when parking and shifting
from drive to reverse, the idle goes way down and sometimes even stalls. Most times it will go down and rebuild itself up again.
I have a new IPC and a new TPS. When running on the road or highway it runs smooth at any speed with plenty of power and pickup. As soon as I shift gears to park the idle drops to just above stalling.
During the cold winter here this did not happen. Just as it is warming up it starts all over again as it did last year.
Any ideas on what is causing this would be greatly appreciated. It is driving, no pun intended, nuts.
Thank you!
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I had the same problem on my 91 crown vic. Turned out to be a combination of the timing being off and the idle set too low. Your Grand Marquis should have a manually adjustable idle. Good luck.

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