'99 silverado 5.3 engin will not idle runs at 2500 rpm and drops down to 1000 rpm the back up to 2500 rpm -------------------------

Link below will tell you the code is a result of the rough running. Use the checklist to find the cause of the rough running and the idle speed should return to normal and the code should disappear.

https://www.obd-codes.com/p0506

Most likely a stick idle air servo. These will sometimes stick after years of varnish in intake. Also can have slight vac leak that it is trying to compensate for. Check computer for any codes especially the instant codes that don't stay long enough to trip a check engine light.

Look for bad connections battery area .

check all vacuum lines very closely for a vacuum leak. might be a small hairline crack that is hard to see.

Not enough info.. is the engine Throttling?? up down up down?? Idle should be say between a "Steady" 600 to 725 RPM.. depending if in drive or not.. BCB..

I would check the idle air control valve that is the what controls rpms

No,only on carburated engines it,idles down to the set point,on yours,it's fuel injected,pushing gas pedal won't kick idle down,should go down after a certain amount of time,does it automatically,If you start and drive a 5 mile ride and stop at a stop sign,it should idle at it's set point if there's no engine problems,which may be.5 to .7 on your tac.When you normally first start,it will idle between 1000 to 1200 rpm's,don't push on gas,let idle say 15 seconds,put in drive and go.

Check out related help links to troubleshoot these problem:----Click the link below:---Idles Fine but RPM jumps in drive mode? http://technoanswers.blogspot.in/2012/07/idles-fine-but-rpm-jumps-in-drive-mode.html
---------Mitsubishi Lancer rpm needle bounces? http://technoanswers.blogspot.in/2012/02/mitsubishi-lancer-rpm-needle-bounces.html
--------How to adjust rpm on 2009 dodge charger SRT8?

http://schematicsdiagram.blogspot.com/2011/12/how-to-adjust-rpm-on-2009-dodge-charger.html
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Engine cuts out at 3000 RPM? http://schematicsdiagram.blogspot.com/2011/12/engine-cuts-out-at-3000-rpm.html

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Cold start problem and vehicle shudder around 2500 to 3500 rpms
http://repairhelpcenter.blogspot.com/2012/10/cold-start-problem-and-vehicle-shudder.html

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High rpm in neutral? Inconsistant high RPM in neutral? http://technoanswers.blogspot.in/2012/02/high-rpm-in-neutral-inconsistant-high.html

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Jetski Sputters and Low RPM? http://whatisbyme.blogspot.in/2012/07/jetski-sputters-and-low-rpm.html

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Fuel Injection Problem And RPM suddenly increases http://whatisbyme.blogspot.in/2012/07/fuel-injection-problem-and-rpm-suddenly.html

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2005 Toyota Highlander: Engine runs hard RPM below 3000? http://whatisbyme.blogspot.in/2012/05/2005-toyota-highlander-engine-runs-hard.html

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1987 Toyota Camry: RPM Drops going Up hills? http://whatisbyme.blogspot.in/2012/05/1987-toyota-camry-rpm-drops-going-up.html

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Cruise Control stops working, Excessive cranks to start the engine and RPM goes max 2500 http://whoisbyme.blogspot.in/2012/07/cruise-control-stops-workingexcessive.html

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1990 Toyota 4Runner: Engine cuts off when reaches 2000 RPM?

http://whatisbyme.blogspot.in/2012/05/1990-toyota-4runner-engine-cuts-off.html

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1999 Toyota Camry: Erratic stalling and high engine RPM at idle?

http://whatisbyme.blogspot.in/2012/05/1999-toyota-camry-erratic-stalling-and.html

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1995 Toyota LS: Misfire at idle or low RPM http://whatisbyme.blogspot.in/2012/04/1995-toyota-ls-misfire-at-idle-or-low.html

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Lexus RX: Tapping noise under Valve Cover At Low RPM? http://technoanswers.blogspot.in/2012/04/lexus-rx-tapping-noise-under-valve.html

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These will help.
Thanks.

May be a cooling sensor issue . If you can hook a scan tool to it and watch the coolant temp pid while idling high it may show a fluctuating reading .

your got a slite vaccume leak