i checked the temps of each cylinder and they were all real close to each other,and i pulled the plug wires on the cyls. in question and the rpms dropped alot just like the rest.i have a very old set of headers hooked to a new 3 inch exhaust could it be a crack in my header or something,the other side cracked years ago when the front was lower i drug it on the curbs many times,it cracked and i had to put a bandaid on it.