A couple of other things to consider about your adjustments with a high idle: is the closed throttle switch working (does the "Off" indicator always appear on the display when you take your foot off of the gas pedal)? Not familiar with this, checked the manual and don't find any reference to a closed throttle switch. Can you shed any additional light on it?
Are you sure that the ISC plunger can retract properly or is it set out too far? It should retract to the point where there is a 3mm gap between the plunger and the throttle plunger at min. air. Yep it is easily 3-5mm back when fully retracted.
Another strange fix which you might not believe, but should consider (this happened on my 85 Deville)...your distributor gear may be worn, and could be causing the car's timing to fluctuate significantly at idle. If you have a timing light, check to see how much your timing mark fluctuates at idle. Easy check, and it'd be good to rule it out. If everything above is in order, I'd really check this item out.
This was the cause on my 85 Deville and the idle was a MESS--it idled high like yours. And, my car only had 30,000 miles! Sure enough, pulled the distributor and the gear was well worn. The slack in the teeth caused erratic timing only at idle which sent the computer into a frenzy. I went through the service manual, and noticed this strange item listed in the "Rough, Unstable, or Incorrect Idle" trouble tree. I'll check it out, would have never considered that!
Seems like you probably don't have a vacuum leak given your test. What RPMs is the car currently idling at in park/drive? I'll double check that later today, but as I recall it was within spec.. 500RPM??
These cars all have on board diagnostics, depress and hold "Off" and "Warmer" on the climate control panel until the display shows all indicators illuminated. Then, the climate control panel will illuminate various indicators to show the status of various things it is reading. For example, the "Econ" light indicates an oxygen sensor rich condition (the light will blink on and off to show it is toggling between rich and lean). The "Off" indicator will illuminate when the closed throttle switch is closed. See the "Driveability" section of the factory service manual (I believe it's section 6).
In any case, if you can set your car to idle at the correct RPM with the ISC disconnected and it drives just fine, I don't think that you have a TPS wiring issue...so hold off there for a minute. First, check to make sure that the closed throttle switch is working and that your ISC is properly moving when it's connected (i.e. do you see it move if you turn the A/C on). If it's moving, and if you've swapped ISCs, that's not likely the issue.
One thing I've learned from having many older GM cars is that the wiring is USUALLY good. Some of the mid-90s cars have ground issues, but that's rare on these cars except at the starter ground.
OK, in which case, I'd really check out that distributor gear if you're getting an idle surge when you're fully stopped. Let me know what you find!
Checked idle and it was a touch high, searched some more and found a vracked vacuum hose. Replaced that hose, sprayed every hose and fitting with starting fluid and am fairly sure there are no more vacuum leaks. I'll look up the closed throttle switch next. Set min idle and have a pretty steady 530-550 RPM idle withe the ISC removed.
The surge was at speed, you'd be driving and have difficulty staying below 35. ISC was extended, buit would drop slowly.
Do you think I should pull the battery cable and let it do some relearning?
By the way (no need to try this if all is OK with your car..but)...it appears you've set the min. air a bit too high. Try setting it at 450 rpms (not 550) or thereabouts, then set parameter P.0.1 to be either 0.7 or 1.1 at this min. air RPM. Then reconnect everything. Car should drive perfectly with no surge at speed.