1993 Pontiac Sunbird LE V6 engine speed became erratic

1993 Pontiac Sunbird LE v6 mileage: 59,000. After filling the tank recently the engine speed became erratic: it would race while driving (tach at 1800 forcing me to keep my foot on the brake in a 25 mph zone); when braking for a turn it would vary wildly between a couple hundred and 1500 rpm; if breaking quickly rpm would fall to under 400, would sputter and stall unless I would two-foot. It continued to do this intermittently, sometimes stalling while cornering. I took it to the dealer for its 60,000 mile scheduled maintenance; they found no problem except a possibly partially clogged fuel injector. I used some OTC fuel injector cleaning stuff to no avail. Took it to a very good independent mechanic. He spent three days checking sensors, motors, valves, computer, draining and replacing the fuel. Everything independently checked out A-OK. He's mystified. What do I do now?