My ford expidition 4.6l v8 triton 99model is experiencing bouncing RPM, 5 to 10 when im using the air-con. whatmay be the problem?

You may be able to ask a Dealer to look at the Vin# ID for the truck to verify. The older 1999 used 3 valves and then the next year or so the engine changed to 4 valves. This was also when the 1999 body style was carried into 2000 for the Heritage Edition. Can not say if the old style body made as a 2000 Heritage had a different engine. The older models did not have "Cam Phasers" which were problematic on the newer generation engines. Sometimes parts stores have access to Vin# requirements in their parts manuals.

check air filter and full filter and spark plugs

check your fuel filter change if nered

Intake air leak

What is the trouble code stored in the computer, and have you checked the idle air control valve ?

Unless I am missing the product years, the 3V 4.6 V8 was not in the F-150 until the 2009 and newer MY F-150s. Up to 2009, the 4.6 L v8 was the 2 V design that was used from 1997 to 2008.

The 3 valve 4.6L V-8 started in the 2005 Mustang and then was introduced to other models.

Hi or low idle it doesn't matter ! This is how you diagnose the problem . Ford Idle Air Control Valve Diagnostics High Idle DTC P1506 YouTube

Need to know the engine size. You have these choices:
4.2 L Essex V6
4.9 L inline-6
302 CID Windsor V8
351 CID Windsor V8
4.6 L Triton V8
5.4 L Triton V8
6.8 L Triton V10
7.3 L Power Stroke V8
6.0 L Power stroke V8
TD

I.A.C sit in top on T.B.I. it's held on by two 8mm bolts, remove bolts clean surface replace gasket and reinstall sensor.