1989 Buick Century intermittent engine stalling or stumblin

Hello: I have just made a donation. I have an '89 Buick Century 3.3L. Just recently we started encountering either stalling or a bad "stumbling" condition. When problem began, I changed fuel filter and used bottle of isopropyl water remover. No help. This is entirely random / intermittent. Does not seem affected by humidity or ambient temperature. Can happen idling in the driveway or out on the road. For example, yesterday I took it out for 3/4 hour, including going down a very bumpy road, thinking I'd jar any loose wires, cracked vacuum hose, flukey sensor, etc. Problem did not occur even once. After dinner, took out again. Within 10-15 minutes, it acted up. Would either stall while driving, or start to stumble badly. If it stalled, I'd try to start immediately but would not stay running. Had to wait 1-2 minutes to start again. If it stumbled but did not stall, I would shift into neutral and coast to the side of the road, and the engine would race for 10-15 seconds before dropping back to idle. Each time it stalled or stumbled, the check engine light would come on. It did not stay on. I jumped across the A and B terminals in the ALDL port and only get code 12.