Have you checked for a vacuum leak yet?

My most common leak was at the vacuum tree at the rear of the intake manifold. As someone suggested, open the dog house cover, crouch on the passenger side floor and look toward the driver's side with a strong flash light. Look for the tree, and see if there are any open ports. Mine would pop off from the back fires caused by improper starting procedures. If I didn't hold the starter on until the engine completely started, it would cough and die, backfiring through the intake, sometimes popping off the vacuum plugs or hoses. I've replaced 2 of the plugs with a hose looped from one for to the other, so if it back fired again, it would pop off one end of the hose. This was better than popping off a plug, which would disappear, and the engine would run crummy for as long it takes you to find a replacement. Sometimes my check engine light would come on as well. Are you seeing that?