I think I'd hook up a meter to the diagnostic port to see if it identifies any codes. A good place to start checking would be the o2 sensor and cleaning the ISC. Add a can of BG-44K for injector cleaning. Check the air filter. Check the ignition timing. Look for vacuum line leaks. Check the oil for gas - 20-30% of air at idle comes through the PCV valve - replace it. Fuel pressure regulator - maybe, but not likely. Many other possibilities - even bad battery connections. Check these and maybe others will have additional ideas if you don't find it on this list.

Good luck,
Frank

'89 [color:"white"]G-Raider[color:"white"] [color:"black"]Supercharged 3.0L, MegaSquirt 2, lockup A/T, 2.5" exhaust, 172k, Cibie H4s/Oscar SCs, Hella Micro DE fogs, Cobra CB, Superwinch hubs, LSD rear/Aussie Locker front, Bilsteins, Lifeline AGM, Rust-Oleum