If your description of symptoms is complete and accurate, then you probably do have a vacuum leak somewhere that hasn't been found yet.
Did you have the manifolds milled while bolted together or separately? The best way is while they are bolted together, but some shops don't like to do it that way.
If they were separated when faced, then you may have to take them off the head, and put a new gasket in between them, and only BARELY snug the bolt/nuts. Install the combo onto the head WITHOUT using a gasket. Snug up the retainers against the head and then tighten the mating bolt/nuts between the manifolds. That way you are sure to have a straight, flat surface for the head-to-intake gasket to seal against. Disassemble it and put a new intake/exhaust gasket on, and do a final reassembly.
If you did this when assembling it the last time, or they were never separated for facing, then all I can say is to keep on trying and looking for the leak.
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David
newbie #4153
