Thanks to all who responded to my last cry in the wilderness. I'm still trying to diagnose the poor running of my B18, rebuilt w/10k miles on it. Here are the latest and most relevant stats:

Bad vibration in drivetrain over 2000 rpm or so, gets worse at higher RPMs, comparable to running on 2-3 cyl's, or a bad u-joint.
loss of power across entire range
Plugs fouling, not coloring evenly...after two hours' driving: #1 is black, #2 is white/gray, #3 is brown, #4 is black.
Exhaust note "looser", hollow-sounding

Here's what I think I know so far: the problem ISN'T
carburetors (fresh HS6s)
distributor, spark plugs, wires
u-joints
manifold gasket (not yet positive, but spraying carb cleaner all round had no effect on RPM)
PCV (removed)

JohnMc, any of these symptoms resemble your recent cam failure? This is a practically new D cam, but I wouldn't rule it out. However: I laid hands on the rocker arms while the engine idled and felt around 1/2" of lift on each, nothing seemed unusual. Valve clearances had tightened before last adjustment, no significant change since then. What's left? Any suggestions would be much appreciated--all best, Nathan