I had the same problem when I swapped intake manifolds. Turned out it was a small fleck of gasket that was left on the head, so it wasn't getting a tight seal. I'd crack the intake manifold back off, and see if that is sealing correctly. Other than that, it has to be something related to a vacuum leak, or the timing belt. Might be off by a tooth, or you didn't tighten the tensioner, so the belt is bouncing between loose and tight...