has the security light been coming off/on lately? When the key starts wearing out the ignition starts thinking the car is getting stolen. Usually though you start seeing the security light randomly come on/off for awhile before the car wont start.
I've found www.naioa.com has a decent amount of info on it. It's helped me a couple times with my '00 impala.
turns out i was a pinched wire between the ignition and the ecm,when it made connection it was not full value the rest of the time the vehicle thaught it was stolen
4 days shop time !!!!!!!!!!!!!!! the owner had a new turn signall switch installed about 3 days b/4 we got it but neglected to mention that till we found it pinched in the collum after a new switch was installed a new pcm and countless hrs looking for the problem we really ate it on this one but it was a good education