The problem might be your Vats security system. This causes the car to either "not start" or "shut down" while running, or start for 5 seconds and then die. (depending on the system) If the security light flashes, i guarantee that this is what the problem is. 1st trick, (if not starting) turn the key forward 2 clicks till right before it would normally start, leave like that for 10 minutes. ( no less ) After 10 minutes, turn key back then forward and your car might start. I did this in my regal for more than a year. If its shuting down while running, im sorry i have no quick fix...
Cheap fix: Bypass the chip in your key. Temporary, but effective. Until the TDM (theft deterrent module) in the dash poops out, then you're kinda screwed. I was told that these types of key security systems have a 7-10 year life span. Sucks for us. This link explains the "Key Issue" pretty well.
Never worry again fix: Bypass the whole VATS/Passlock/passkey I or passkey II system entirely. Literally take it out, and throw it away. I just did this in my Regal. Took about an hour. Took longer to find the thing than it did to fix the problem. For a list of cars effected by this problem. See the NEWROCKIES link on the right side of their page......... They sell a unit for 200 bucks to totally bypass this system FOREVER. life time-money back and all that. Finally did this in my car. Well worth the 200 if you've looked into getting this replaced... Anywhere from 600-1000 bucks.
----- With this fix, you can take that whole Vats, passlock, passkey **** system out of your car all together. Good info on both sites