I had a very similar problem to yours on my TBI 454....and my high idle problem (after replacing everything, computer and eprom included..alot of $$$), ended up being a loose intake manifold. 8 bolts on each side of a big block...and on the drivers side, 3 of the 8 were loose, 2 were so loose I thought the ratchet had slipped to neutral when I started backign them out...the rest were snug. MY #3 and 5 plugs came out WHITE...while the rest came out black....
After replacing the gaskets, the difference was instant...im still fighting a running problem, but at least it idles. Cranks up...goes ot 1300...drops after a few minutes to about 1000, and HOT, to 850ish, and 725ish in gear HOT.
Ive got a new Delco TPS, IAC and CTS coming for good measure, I used cheapy "standard" parts when I redid it all last year...better safe than sorry.