Why does my car feel like it wants to die on me? I have a 1999 Pontiac Grand Am SE 4 cyl.?

My car has been acting funny for the past 2 weeks and it finally cut off on me while i was driving. So i took it to a shop and they said it needed a tune-up. I told the guy i just had a tune-up done in january 2007 but he said the spark plug harness was melted and he had to change the heater coil. So i said fine and i ended up paying $438.40 for everything. Well i brought it home and nothing has changed it still dies when i try to accelerate a little and you can feel it while its idling forcing itself to stay on. I think its the oxygen sensor after the catalytic converter because a code came up back in november 2006 when i took it to autozone for the check engine light that came on back then. I did not have the money at that time to change the sensor. Can someone tell me what may be wrong with my car or tell me what exactly does a 1999 pontiac grand am SE do when the sensor goes bad? Does the car die and lose power when the sensors go bad? or is it soemthing else that is wrong? HELP!!!