[Brian]

Back in the '70's, I remember reading an article in a NZ Hot Rod magazine about the editor having trouble getting his flathead to run sweet....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! Now, i dunno if that is your problem here, but that article has always stuck in my mind, and I always grease that gasket when fitting.