high idle on '99 C2

[craigmakes3]

My '99 c2 has an issue that is driving me nuts. When driving, the idle of the car does not drop below 2300 rpm unless I leave it in gear and brake down to 1000 rpm or so, and then it will idle normally. As soon as I get back up over 2300, it not drop below that level, even between shifts. Problem seems worse when the engine is warm. Here is what has already been eliminated from the list of possible causes.
1) Air idle valve, replaced after finding crack in original
2) Mass air flow sensor - came up on CEL readout, so i replaced
3) Throttle Cables cleaned, checked for binding, junction box cleaned
4) Throttle body cleaned, spring checked
5) Smoke tested for vacuum leaks, found two and repaired.
6) Throttle pedal disassembled, cleaned and lightly lubed
7) Manometer reading at oil filler checked

None of the above has made a difference, if anything, it has gotten worse. I have posted quite a few threads at Rennlist, and I need to spread the net a bit wider to see if i can get lucky and catch the solution here! Several Rennlisters with the same year car also have the same problem. Eternally grateful to the sage who solves it!