stalling

I'm having a problem with the one I recently purchased and wondered if anyone has encountered this and found the solution, as it seems a mystery to anyone trying to diagnose it. 3 times now the car has stalled (well, twice attempted to stall). Each time it was after the car had been driven maybe 50 to 60 miles give or take. The first time, I was at a stop and the engine started to act like it was going to stall. I gunned it and after some hesitation, it went and seemed OK. The second time I was driving up a rather steep long grade and my Trac Off light came on and I suddenly lost major engine power. I coasted to the shoulder and turned it off. After a few minutes I restarted it and went a couple of miles before having to stop to cross traffic... and it stalled in the intersection. Before it stalled all my dash lights dimmed out, came back on and it just died. At that point the battery, oil pressure, trac off, and service engine soon light were on. The key locked up so I couldn't try to restart. After a few minutes I started it and pulled into a parking lot. A guy pulled in next to me to help and he thought the engine sounded like it was choking on fuel and assured me it wasn't the alternator. After sitting there for maybe 2 hours, it started up and I got home fine. Before I left tho another guy said it sounded like alternator to him. So anyway, I go to Autozone and they run the code check and it comes back "system too lean" and they said I may have an O2 sensor going, but at that point the Service Engine light went out and they said wait till it does go for sure. The alternator and battery tested out fine and they said the O2 sensor wouldn't affect the lights and stuff like happened or cause it to stall. Of course it was running just fine at that point. They said they doubted a garage could really give me much more information than that. Someone else suggested changing the fuel filter. Does anyone have an idea? I really love this car, but I'm worrying now that I'm in for more than I bargained for.