Although no one seems to have had the same experience I reported in a couple of posts a week or so ago, I managed by trial and error to narrow the source of the problem down to the O2 sensor. Simple disconnection of the lead wires resulted in immediately smoother and normal performance. So I picked up a new sensor, installed it, and everything's running fine now. Since the sensor had been replaced by a garage that does emissions tests only last August, I'm planning to give them a call and inquire after the length of guarantee on the sensors they install -- but I won't be holding my breath. Although my pleas for help failed to generate any helpful tips or suggestions, I thought someone might benefit from this account. Cheers.