1991 Pontiac 6000 Got taken at car lot trying to make lemon

Engine Performance problem
1991 Pontiac 6000 198000 miles

So we bought this pontiac 6000 at the car lot yesterday. Within 2 hours the car is stalling in traffic. It starts right back up but keeps stalling while driving nearly causing accidents.
What could be the problem?
Of course it didnt do this on the test drive.