Anothe "High Idle" question

I have a good friend with an 03 GT Ive been working on for him. Since I installed the O/R H-pipe the car gets a high idle after awhile. Seems to take some driving and on/off cycles to make it happen again after I reprogram the puter. It goes to 1600-1700 rpm with the clutch in and then goes down to 1400 rpm sometimes 1200. I have the idle set at 620 in drive and 620 in neautral (its a stick though) and when I check the tune its still set at that. Ive cleaned the IAC and checked for vacumme leaks by spraying starter spray (I know) around the plenum gaskets and lines and there is no change.
He dosent think it did it before the H-pipe but I also retuned the Diablo to turn off the rear o2s and MIL at the same time. Im lost I know now its the puter doing it cause if I unplug the IAC itl go down to 450~rpm. I can set the t-body for 600-650 but since it didnt change the High idlke problem I put it back to where it was. And the TPS voltage is .90 volts at idle. Also there are no trouble codes.
Im thinking telling him to get a new IAC cause mabey his is hanging and staying open longer than its told to

Mods:
C&L plenum and pipe
T-body (dont know size or brand)
MAC O/R H-pipe
Basanni Cat back
NX wet kit (did it before the kit)
Diablo tuner
4.10s

It has other stuff but nothing for engine/drivetrain.

Please guys I need a new direction

Is there anybody in there?
Just nod if you can hear me
is there anyone home

Can be for a more agressive throttle. But .90 is spec from what alot of people on here told me. It was .95 and I left it set at both to see and the problem is the same. Im gonna go drive it myself today and try to get a better feel for it See if the RPM numbers he gave me are accurate.

Yeah, I used starter spray. Always worked before Its not vacumme, the puter tells the IAC to open more after 50 or so miles after I retune it. Im thinking the IAC is sticking on teh open stroke and is "hanging" open too long but why would the puter do it later and not emidiatly. And it seems if the computer saw the high idle from the IAC pulse it would shorten it cause it does have an adaptive program to it. I have heard sometimes Diablos do this and am also thinking of returning the tune to stock and see if it still does it. I will see him tomorrow....er today and try it out. Couldnt hurt ......as long as we keep the nitrous off