Late last week the engine started misfiring, so I made an appointment to take it in today. One cylinder was having problems even when its spark plug was swapped with a good cylinder's, which meant replacing the ignition cassette, which is what a Saab uses instead of a coil and distributor — it's essentially one ignition coil per cylinder, plus timing sensors and so forth. Wasn't a cheap repair but I saw it coming.
The good news is that it resolved a lot of problems which I thought were just the miles adding up -- some sluggishness and roughness accelerating and decelerating are gone, and some of the vibration is gone too, so it sounds as though I've had a timing problem for a while.
It's one more breath of life into the thing, at least, so I think I'm going to stick with it for the time being.