I have a 99 4.2L and i've been having a relatively small (but annoying the hell out of me) issues with my idle.
Every ten seconds (like clockwork) my idle will jump from about 850 RPM up to 1000 to 1100 RPM then drop back down.
I changed the plugs and wires last year and haven;t put much mileage on them (probably around 8K kilometers)

I'm not sure if this engine has an idle air control valve like the old 2.3L or 5.0's that im used to dealing with it. To be honest i havent had time to look. i will when i get home but i wanted to pick some people's brains first.

 

Yes, the 4.2 has an IAC.

It's not due to the AC compressor cycling on/off is it?
Sure sounds like that could be the cause.

 

yeah sorry didnt throw an update in here. the ac compressor is cycling on and off. im going to evacuate the system and pressure test it tonight. if worse comes to worse ill replace the compressor seal, fill it and see what happens.

 

well im pretty sure the coolant must have just lost its charge, i pulled it all down and wasted it (my dad had put some of those redtek kits in it a while ago against my wishes and i didnt want to contaminate anything) when i was pulling it down i noticed it pulled down to 400 microns so i didnt even bother pressure testing it. filled it with 33 oz. compressor no longer cycles and AC works amazing well now

 

Good deal!
Thanks for the update and glad you got it all squared away.