Brian>>tried everything to no avail, culminating with removing the intake manifold and applying grease to both sides of the gasket and rebolting it down which cured the problem!>>
Also a good possibility. IMO, smearing wheel bearing grease on all 4 gasket, block & manifold surfaces is where it's at (except exhaust). 
As a quickie test for vacuum leaks --- when engine seems to be missing, try spraying WD-40 or smearing grease on manifold edges around problem cylinders to see if it tries to smooth out.