My 245 is finally back on the road, after a maddening 6 months attempting to diagnose a severe off-idle hesitation and an intermittently rough idle. See the old thread:
http://www.brickboard.com/RWD/volvo/1376979/220/240/260/280/offidle_hesitation_rough_idle_misfire.html

Turned out to be something very simple: mis-adjusted valves. The machinist who refurbed the head last July apparently was confused or made a serious math error when he readjusted the valves. Since the machinist does lots of Volvos and has done work for my mechanic for many years, I trusted that he'd done the valves correctly. I spent many hours looking for faults in my own work and for faults in the fuel and ignition systems.

The odd vacuum readings I noted in my initial post did indeed point precisely at what was really wrong.

John