got a 91 Chevy Z28 ,engine idles rough @ 2 or 3 seconds interval ,when turn on the AC it idles ok , or after i drive it for 30 minutes or so it idles ok too, my friend told me it could be the compression, there is oil leak @ the top back end of the engine. if it's a compression issue does car should have rough idle all the time

 

check the fuel filter. sounds to me like it might be on its was to being clogged. at idle its not pulling enough fuel to idle smooth but when the ac is turned on it raises the idle or when u drive it there is better flow. I'm not master tech but it couldn't hurt to check. and hey if I'm right... thats a cheep fix
and dropped compression on a cylinder would cause a "miss"
constantly no matter what rpm

 

thank you Mick