Well, all's well in my personal Volvoland.

Before I opened the fuel distributor etc, I did a thorough check of the vacuum lines... the big one to the brake booster had a crack - it had "merely" deterioriated - right at the manifold nipple, half way around the hose. Hmmm, I wonder if this might possibly be a non-standard configuration..??

As you might guess, following replacement of said hose, viola! all is well. Idle has returned to a very happy and comfortble 800 rpm, and the engine speed actually drops between shifts!

Amazing... maintain the machine as its designers intended, and it still works. Huh, maybe there's a lesson in there somewhere (how many trips have I made aroung THAT mountain...).

The heater/defroster vent weirdness (hope you don't have that phrase copyrighted, Dave!) is still there; it appears that the little valve is defective (based on the suck/blow test).

Thanks to all who responed. But the grand prize goes to Art for being the first to accurately diagnose the problem.

AS always, thanks.