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Idle bounce at traffic lights

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I have a 1999 K2500 Suburban, 5.7L engine with an idle bounce problem. The idle bounce only happens when the following conditions are satisfied:


1) Transmission in Drive and brakes applied. (as at a traffic light)

2) A/C on (or anything else that uses the compressor, like defrost).

3) Engine fully warmed up (has been at operating temperature for 10 minutes or so).

4) It's warmer than 70 degrees outside.


The bounce won't happen or stops if any of the above aren't true. (Say, if I were to shift from drive to neutral or park at the red light., or turn off the A/C.)


The vehicle has 75000 miles on it now. I bought it last summer with 65000 miles on it, and am unsure of prior maintenance history. Recently, I had the injectors and throttle body cleaned, and the IAC valve replaced.


Thanks for any help,


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My dad's 98 pickup (5.7 vortec) did that after about a year, mostly when the a/c was on, idling in drive sitting still. The RPMs would bob up and down. I believe they flashed the computer with some new software because it was a computer screw-up. I think the comp was misinterpreting feedback from the transmission pressure and compensating with throttle. it bobbed anywhere from 500 -900 RPM.


Initially thought it was fuel filter but there was a service bulletin on it.

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