Bone stock '90 T/A auto with L98 5.7. So I start it up and it runs fine smooth as can be. Then once it warms up just a bit, the idle fluctuates downward , minus 100rpms, then up to normal then down. At a stop, in gear, it fluctuates enough to stall the car once in a great while, and going down the hiway with cruise on, when I head down hill so there is very little gas being applied, it fluctuates noticeably, like a surge, but the rpms surge down losing 75-100rpms. No codes are coming up, not the slightest of a miss or stumble at any other time, no hesitation on acceleration,power and fuel mileage are good, any ideas, i put in new plugs, cap, rotor, I run 91 octane, it has 103,000 miles, clean oil,

 

I'd start by checking the throttle position sensor. It's a sticky here somewhere.

 

yep, put a new tp sensor in and adjusted it, cleaned and adjusted, and reset the IAC and its housing, can't find any vaccum leaks, replaced the gaskets in the throttlebody up front. it set for a year, we jump started it and its been doing it evry since.

 

Take a timing light and put in on each plug wire as it's idling poorly. See if it flashes evenly and consistently on all cylinders. The distributor pickup coils can go bad and cause a hunting idle as the ECM tries to compensate for the misfires. Could also be a bad wire or fouling plug.

If that looks good, take a long screwdriver and put one end on each injector and the other on your ear. See if they are all firing evenly.

 

observation update: idles smooth at 750 in park, push on brake and no change, shift it into drive and it immediately starts jumping from 525 to 675 in a consistant 2 sec rythm, put it back in park and it smoothes out, making any since to anyone??

 

In park the engine is under no load. When you put it in gear, the torque converter puts a load on the engine. The load pulls the RPM down, the ECM detects that and makes adjustments in an attempt to stabilize the idle. In your case, it can't find a set of settings that makes it stable, but it keeps trying.