Hi folks,
This has only recently started to happen.. my car is a 1990 Accord
EX, Auto, with 178k on the clock. The only work I've performed on the
intake was to clean the EGR ports(about 5months ago), and routine
throttle-body cleaning.
This only happens after the car has been warmed up and out on the
highway. The rate of incident is increasing. After letting the car sit
and warm up(very cold in PA this time of year) for 15-20mins, idle is
fine, drives fine.

After driving, when I put the gear into park, the idle will jump
from 1000 to 1500rpm(will raise to 1500 over about two seconds, then
drop sharply to 1000, and repeat). Putting the car into
reverse,D4/3/2/1,or neutral returns the idle to normal.

It's almost like when I shift from Park to Reverse: the idle raises
for a second, presumably because the reverse gears are kicking in(???
this is just an educated guess ???).

The gear indicator *does* indicate it is in park. I have tried
shifting up/down through the gears while the engine is off to possibly
clean any bad contacts in the shifter unit. No good.

I have unbolted the pulsed-air valve(on the intake manifold, the one
with the small metal screen in it) and cleaned it with throttle-body
cleaner. I also inspected the cold-start air valve per service manual
directions(has the wax bi-metal and coolant lines in it to idle high
when cold). I took the top service cap off, noted the air flow while
cold, waited for car to warm up, and then observed no air flow(normal
operation).

Any ideas?

Thanks folks!
-Jamie