I have a 99, 2500 Suburban with a 454. Sometimes when I stop and have the truck in gear the idle rpm's will drop a couple of hundred and then surge up about 400. Then it idles normal for a second and starts the routine again. I have had different mechanics look for the problem and they can't find it. No codes show up either. Changed a lot of parts but it still does it.

Anyone have a clue?

Plug wires? Had this with a '94 454 and plug wires fixed it.

Idle air control motor.

sounds like an EGR valve

you can get an EGR gasket from NAPA that has a screen in it to catch any particles that may pass through it and make it stick open.

Check your fuel psi for any leak down.....just hook up a guage to the fuel rail and energise the ign system...the fuel pump will prime and should be close to 55-60 psi.....then turn it off and sewe it it holds.....if not then the regulator is going bad....very common on these engines.

another common problem is the crank pos sensor....easy to change too but most times if it goes bad the truck wont run.

my truck had some goofy acts when i had it...ran like it was missing on one cylinder...my cat even glowed red once! we did all sorts of tests.....i ended up changing the igntion controll modual on a hunch....and the symptom never came back so i can only guess there was a problem with it.

If you need any parts the best places ive found are www.gmpartsdirect.com if you know the GM part numbers or another is www.lmctruck.com

Roger, had a similiar problem several years back. The idle would go way up, like to around 1500 rpm, then after a few seconds go back to idle. Then after a few more seconds do it all over again. Nomatter what you did, you could'nt regulate it.
Turned out to be a bad throttle position sensor. Might want to check for that if nothin else helps.

clean the throtle bore !
clean it avery 12.000 miles and you wont have that idle problem again.

Just saw a 5.3 doing the same thing. It had a fuel pump dropping pressure while it ran-took about 5-10 minutes of idling before it started but the pressure would drop to 30 psi and then go back up, idle would drop, then surge back up. Just hook up a fuel pressure gauge and watch it for a little at idle and see if it stays steady.

I've changed the wires, fuel pump, fuel filter, intake manifold gasket, idle air control motor, distributer cap, plugs, cleaned the throttle body.

Frank, I'll try the throttle position sensor next. Your problem sounds like what my truck is doing. If you put it in neutral did the problem go away? Mine does.

Thanks everyone for the replies.