Help please, Im stumped.

[fuzzypuppy]

Ok here goes, I have a 91 GMC 2500 w/ 5.7 4l80e.
The problem is idle, starts fine, runs great, starts w/ high idle just like its supposed to, but once it warms up, or after a good run on the freeway as well as just surface street driving. the idle goes way down, 'it never has stalled' but it acts like its thinking about it, ive had an o2 sensor go bad about 6 mos ago and its not quite that bad, ive pulled and checked the egr, fresh wires, cap, rotor, plugs, I thought about fuel filter but it pulls good on demand at higher rpm's, and i figure this is where it would be worse if it were indeed the filtertiming is dead on, i work on my own vehicles, always have, for better than 20 years. im just totaly stumped on this one. i did put a mild edelbrock cam in about 8k ago, but again since it still pulls through the rpm's just as normal i ruled that one out as well, checked for vacume leaks used fuel injector cleaner as well with no luck,tried everything i know.
i have not made any recient changes to the truck in 3 mos to lead me to think down that road, gas milage has not suffered either, just a very low idle once warmed up, as we'' the exaust does not smell like its too rich, lean if anything, but i can't be sure on that one. the guids are bad but even with that i only lose about 1/2 a quart between oil changes.
the 350 has just turned 170.000 mi.
if anyone can give me some clues as to where to test next i would greatly appreciate it, i much of the time put about 300 miles a day on the truck through the harshest deserts here in sothern cali and need to be sure its in good condition.
thank you for paying attention to this long post. as well as for any info on my problem.