My car wont stay started?

I have a '91 oldsmobile, 88royal. It started with a brake light out last week. Yesturday the car would stall when I slowed down unless I threw it into nuetral and back into drive when I wanted to go. Today we drove to the store. When we went to leave the car would start but wouldn't stay started. You can hear the engine turn over but wont stay one for more than a second or two. I left it there.