Car won't run
 

I just bought a 1999 Grand Prix V6 3.8, we had it about 3 weeks and now it won't start or run. When you go to start it, it turns over for about a second or 2 and then quits. Sometimes it will do just fine. The car has 122K miles on it, so I expected problems to arise, but what could this be??? It doesn't do it everytime you get in it. My brother-in-law threw it in reverse when it started to die and it started running. Any ideas???[&:]