Symtoms:

1989 245 LH 2.4, w/951 ECU and 169 ICU, automatic, starts fine every time, hot or cold, idle punches up on start to about 1100, then settles right in at 750 RPM, so far so good

The following happens with regularity. Almost, but not every single time:

-after driving at cruising highway speeds, then coming to a stoplight after an exit, vehicle will idle at 1000 RPM instead of the usual 750 RPM.
-At that point, if I put transmission in Park or into Neutral, the tachometer will immediately race up to exactly 1500 RPM.
-If I then put it back into Drive, it engages into gear back at 1000 RPM. No transmission shifting kicks the idle back down
-Tapping on the gas throttle does nothing to bring idle down
-If I turn off the ignition, then turn the vehicle back on again, it seems to "clear" itself, idling at 750 RPM again until the a similar highway speed to stop scenario repeats itself
-idles a tad choppy at 750 RMP, but that could be other issues (injectors, oxy sensor, timing?)

What I've done over last four months:

-Idle Control Valve tested with 12v and open/shutting properly
-New plugs, wires, cap, rotor
-Breathing system thoroughly cleaned
-Throttle body cleaned, TPS switch checked and properly adjusted
-016 AMM seems to be working, but I don't know how to test it
-All engine ground wires checked
-All ICU, ECU connections check for snugness
-Any time I do work I reset the codes and pull the 25 amp main fuse

Things I might try:

-Reinstalling the pink 561 and original ICU (can't rem. #)

Please help this has gone on for a long time. I cant think it's the missing RPM signal (3-1-1)? It must have something to do with the idle speed control? I'm flummoxed and want to fix this!

Thanks,
Jason