A few vacuum lines that some dummy named Dave has been overlooking for the past 4 months or so were likely the biggest problem.
We were about to get it smogged last Friday when it really started running kinda funky, skipping around a bit at cruise and idling really lopey. Turns out that the line going to the MAP sensor was rotted apart, and the line from the underside of the upper intake plenum down to the charcoal canister was pretty spongy and leaky-looking, so I swapped both of them. I never noticed them because they were so far out of sight (the MAP sensor's line was hidden by the A/C hose going over it) and it was only the ends of the hoses that had rotted - I felt the other half of the hose and it had always seemed fine before, so I stupidly assumed that the whole hose was good. Also found and fixed a couple of vacuum leaks on the Capri while I was at it, so now it's all runnin' perky, too.