RT, have you adjusted both the mechanical idle stop and the idle bleed screw?

The former being at the front of the throttle body and acting upon the butterfly linkage, and the latter located on the firewall side of the TB.

The RS I picked up a few weeks ago wouldn't idle due to having compression issues (burnt valves). I adjusted the air bleed and it idled well enough to be driveable.

Can't recall for sure but I think you'd back out the screw to get more air thru.

You're definitely on the right track since popping a vacuum hose speeds up the engine. Obviously opening the throttle butterfly accomplishes similar and that's how most EFI motors are speed-controlled anyway.

All the IAC on a GM TBI-equipped vehicle is (for example), is a motor-operated valve which opens or closes to admit more or less air to adjust idle speed. I forget what type of IAC the 2.6 has but you may have issues with that.

It's possible you have other underlying problems but you may be able to at least compensate with the idle bleed.

BTW if your compression is dwindling due to the increased miles, it certainly can throw things out of whack. A compression test might be in order.

HTH & G'luck........ed