It's either a valve problem or a blown head gasket.

A blown HG may not show up on a compression test, but will leak when subjected to firing pressures. firing pressure is a whole higher than cranking compression pressures.

And yes, a partially blown HG can leak just in on direction at first. Number 6 might be leaking into the next cylinder. If that cylinder has the intake open when 6 fires, then there is your stumble and wavering vac gage and no idle problem.

If it's #6 intake, and not the HG, then it will cause the same problems.

Let me add that burnt and leaking ex valve can make your vac gage waver a little, but not alot like a bad intake would. If the ex valve is bad, then engine vac will suffer when the piston for that cylinder drops on the intake stroke. It will cause a slight wiggle in the gage needle everytime it happens. But a bad intake valve will really make the needle dance cause firing pressure is blowing past it back into the intake manifold.

About all you can do is pull the valve covers and check valve timing and lash. If all that is ok, then pull both heads for rebuild and pray that the rings will hold up after you screw tight new valves back on. Either way you will be replacing the HG's with new ones.