Not sure if it's a big deal but if I have my car in park/neutral and rev up to about 1300, I can watch the needle drop and pick back up even when my foot is steady. It does the same thing if I'm at a redlight or something and have my foot on the brake. It feels weird almost like a misfire.

 

set up the coil wire so its close enough to the terminal on the cap for the spark to jump. with the engine running see if the spark is steady. if so your either too rich or too lean. black smoke says too rich. pull the plugs and see what they say. and go from there.

 

Vacuum leak maybe?

 

Vacuum leak, IAC, or injectors.

 

Ok I'll clean my IAC valve and I think the running rich idea might be it considering I have headers with an exhaust leak.

 

Good responses. Weve also seen issues with timing control cause irregular idle. Check with a timing light for unusual changes in timing. It may be the ign module.