At the mechanic's suggestion, I replaced the oxygen sensor -- problem fixed! Seems the oxygen sensors, if they get clogged, or when they get old, respond more slowly, especially at first, and confuse the system. Thus the erratic running when it started. $30 later, it's just fine.

Tip for anyone replacing oxygen sensors on this (and probably other) models: go to your local flea market and buy a 7/8 inch (for U.S. market -- it's not a size that translates to metric, and I don't know if metric-market cars use a metric-size oxy sensor) box wrench. Estimate clearance from the oxygen sensor to the fender wall, and cut off the excess. I used a carbide grit blade in a reciprocating saw, but you could use a carbide grit hacksaw blade too. It makes all the difference to have the right length wrench for the job.