"This is the frustrating mystery of it: I have tried everything from the TH fix to cleaning and then replacing the IAC valve, all to no avail as far as this "high revving" thing is concerned."
Have you cleaned your intake/removing them?
I could be wrong (I'm just repeating what I read in a SVT forum awhile back) but maybe one cure (TH fix) is actually having an adverse effect on the idle problem, since curing the TH will probably defeat the original purpose of why our cars were designed with the slow return to idle characteristic.
I've been thru this myself. TH fix, cleaning everything, replacing IAC. Pretty much solved my problem, but still had a few hot idle issues -- unfixed the TH fix and it now works exactly like it did when the car was new.
