i noticed this on my 96 xj6 the other day. when i started the car in morning. it idled fine but after driving it for about 10 minutes when i put it in park, the idle races to 1500 rpms. when i put it in reverse or drive, the idle drops back to 800. It does have to do with park because i sat there and did it 5 times and it does it right when it goes in park. the car only has 38k miles on it...

 

My first best guess.

1) The idle control valve (ICV) is sticking, common in teh USA market cars.
2) The Exhaust Gas Recirculator (EGR) valve is stuck, again common in the USA spec cars. We missed that thing down here, luckily.
3) The throttle body is all gummed up with GOO, common world wide.
4) The throttle return spring is not doing what it should, common in the early life of these cars, most have been sorted by now, but yours may have missed out??.

There may be more, but that will keep you occupied for an hour or three.

 

thank you Grant. all very good ideas. I will try them all.