[gadget73]

have you tried closing off vacuum lines one at a time while watching the gauge? I've seen cracked plastic vacuum trees on the firewall and bad brake boosters cause problems. Improperly de-smogged stuff can also be an issue, there is usually a line between the smog pump and a T for the cruise control. If that T isn't removed, its a big leak. Same with evap purge hoses if that system is removed but the line isn't plugged off.

Also, have you ever done a leakdown test? Last motor I had low vacuum on for no obvious reason had lots of leakage and a cam that was a tooth off.