The culprit? The lousy drain valve on the filter. Dribbled out the top, couldn't see it due to the trail of gunk on the filter housing (no doubt caused by the dribble...). Lost prime, allowed lots of air in to the system. After replacing the valve and cracking loose a few injector lines (something I have NEVER had to do in nearly 20 years after changing a filter to bleed some air), it came to life. What I could not figure out was why the lift pump was acting up - as if it were dying or had a voltage problem.