Hi Jack;
Is this the 164 engine still? I probably got mixed up about the previous threads.
A fluctuating vacuum signal is a sign of a valve hanging up a bit, or a broken spring, or a weak spring. Roll the engine to TDC number one compression. The number six cylinder should be on overlap, and the amount of overlap must be even on both sides of TDC. You can rock the engine either way and see the valves on six open and close, equally in both directions. That will prove the cam timing is correct. Grab all the springs with your fingers and rotate them (when the rocker is not on them) and check for equal tension on all. If you want, just pull the rocker shaft and with a rag on the palm of your hand, depress the valve spring. You will feel a weak one for sure if it is there.
Plug all vacuum ports on the manifold, including the brake booster hose and heater controls.
Did the manifold have secondary throttle plates in it? Spray WD-40 on the shafts to see if one is allowing air in.
When you find the problem it will be a simple fix.
I have never seen a Volvo casting break through. Salt water cooled marine engines yes, after many years, but not street engine.
Good luck and best of the season.
Rhys