I have a 89 model with v6 and my problem is when its started cold it idles around 1500 rpm's. but when it gets warmed up the idle drops to around 500 and even lower with the ac on. i'm not sure what to do to it. my sons 92 is the same way. i have adjusted the idle with the slotted screw on the side of the throttle body but doesnt seem to help.
what do i need to do? they both are automatics don't know if that matters or not. they both idle to high when started cold and idle too low when warmed up and when the ac is on its worse.

 

the high idle when cold is normal. your about 200 RPM low at temp though. the throttle adjustment is between the throttle body and throttle linkage. it is a threaded rod with a stop nut on it. also before going that rout you should try cleaning your throttle body.. also a new o2 sensor helped smooth out my idle quite a bit..