Idles high after going from park to gear and back.

[MadRatt]

Hello,
Its been awhile since I've been on here. I figured the experts here are my last hope to solve this issue.

So I have a 1989 c3500 7.4L with a 3sp th400. It has 116k on it. It is set up as a wedge ramp single car hauler I use for personal vehicles. I am having a very unusual problem. It starts in park and idles perfect at 750rpm. If I go into gear it idles down to around 625rpm and then slowly starts rise up to about 750-800rpms. when I shift back to park it will flash up to 1700rpms then slowly over a minute settle at 1200-1300rpms. If I shut it off and restart it. It idles perfect at 750 until shifted into gear again.
I have done these and replaced the following:
1. New IAC and have done the relearn many times while diagnosing and disconnecting the batt.
2. New TPS set to .62
3. rebuilt TBI
4. New base gaskets. (both of them) The BB has the base coolant chamber to help warm up the TBI in cold weather
5. I have done the base idle set procedure. I have set it at 500rpm and it still ends up at 1100-1200 I have also set it to 625rpm and it still settles at 1100-1200 RPMS.
6. Replaced the egr control module. The egr valve tests fine.
7. new fuel filter
8. Cap rotor plugs and ignition module. Wires are MSD less than a year old
9. Capped off IAC and it Idles fine at set base idle. So there is no vacuum leak.
10. new pcv
11. New ecu
I am down to thinking It has to be something in the Prom? Telling it that it should be at that Idle. I don't want to just throw more parts at it at this point. It runs great down the road but at a light it starts to get warmed up because you're fighting that high idle with the brakes. Also it revs up to 1700 when you stop and park and its a bit embarrassing at a car show. Everything I have listed that I have replaced was a neglected part from the previous owner. I've had the truck about 5 mos. So I didn't mind replacing those things and each one made the truck run smoother but never fixed the idle issue. I've several friends that are gm mechanics and they just scratch their heads like its possessed. lol
I have no problem spending the money to make it right just don't need to waste money on nonsense. I have checked everywhere I can think of for vacuum leaks and there is none.
So I'm down to PROM. If the PROM carries the idle info, could it have got corrupted?
Borrowed a scan tool and you see the IAC counts changing and hear the idle change accordingly.
Also the p/n switch shows its working in the scanner.

Any info would be greatly appreciated.
Thanx in advance for any help given.
CJ