91 Firebird suddenly hasitates badly

My 91 Firebird, 5.0L HO, 2bbl TBI, 134K mi, has suddenly started hesitating badly. It starts easily, but not quite as quickly as normal. In neutral
idle, sometimes it runs smoothly, but other times, it alternately revs then slows in about a five second interval until it stalls completely. It will usually rev up okay though. In gear, with light accel pressure, it seems to miss about every other fire. With heavy pressure, it barely fires at all and barely accelerates. I can get it to accelerate gradually by rapidly pumping the pedal. All seems to improve when engine warms up, and it eventually seems to run nearly normally. There is no backfiring nor gas smell. New fuel filter made no difference. New plugs last year. There are no ECM failures indicated, but on first check, an O2 sensor code came up, though this event could be years old. In field service mode, it indicates open loop at first for a few minutes, then closed-loop with mostly lean indication. Advice?
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