This problem just started last week.

The engine starts fine, races normally when started then idles down to a normal rpm as it warms up, and as long as I'm running (in the first "D" position of course), all is fine.

But when I get to where I'm going, or simply stop along the way somewhere and shift the lever from "D" into "P" (Park), the idle drops usually low to a point where the engine runs quite rough and stalls within a few seconds.

The engine starts immediately upon turning the key to the START posiiton and has the idle rpms returned to normal.

What would cause that?