I've got an 88 GXL, I bought it a year ago and it ran great till a few months ago. I don't think it had all the power it should have, but it ran clean. A few months ago, i noticed that on a cold morning when started, the idle would oscillate between 1500 and 1700 rpm. After i tapped the gas, the 15 second warmup would occur and then it would return to normal idle. After a few weeks, the problem got worse.
What happens now is the car will start up, rev to the 3k warmup for 15 seconds then drop down to 1500 and oscillate. if i tap the gas the idle will go back up to 3k for about 5 mins until its warm. then the idle goes back to 1500-1700 oscillation.
I have checked the TPS and BAC valve, both are in spec. I have checked for vacuum leaks everywhere. I recently had the UIM removed, so I took that back apart and made sure it was sealed w/ RTV sealant. When listening for other vacuum leaks, I hear a hiss on the right side of the dynamic chamber, but i cannot pinpoint it. If i listen through a vacuum hose, it seems as though im just hearing air passing through the large hose connected to the right side of the dynamic chamber. I have constructed a code reader with the two LEDs and the car does not give any codes. I've checked the voltage of every pin on the ECU and all components are to spec, except for the AFM voltage at idle. I've read threads regarding the AFM and it seems as though the symptoms of a bad AFM are more severe than what I have. Is there any way that I can test the AFM? The FSM says to take it to a dealer.. which im trying to avoid.
Thank You for any help

 

have you checked your ACV, I have had some problems and herd of many more due to a bad Air Control Valvue. Check it out and that might be that.