I had a very similar problem and I replaced the manifold because it had cracks in the rubber...and somehow that fixed my problem. Apparently even tiny little cracks cause problems.
Ok, here's what ya do for those tiny cracks in your manifold. Just take a can of black Plasti-Dip and put 3 or 4 coats of the stuff on it. problem solved. That stuff is like a gift from heaven. It will fix all kinds of stuff.
If that stuff can take the extreem heat thats generated inbetween the cyls,tollerate vibration too and you can also coat the entire intake manifold with it still installed on the motor sealing up small cracks leaking air leaning fuel mixture it could work.
But for how long being exposed to so much heat/vibaration,etc,who knows?