one day it will start next day wont

The problem on my 2003 Grand Prix GTP was the Camshaft Position Sensor. The engine was dying in stop and go traffic. It also died when I'd come up on a red light and take my foot off the gas. The CPS degrades over time from heat issues. It doesn't throw codes when the car dies. There is no test for it. The car runs after a restart but can die in traffic without warning. I had the CPS replaced at an independent shop because the dealership couldn't see it throwing codes, which it doesn't. I basically had to tell them I had already determined what the problem was and I just wanted them to replace the sensor for me. They did and it has run like it's brand new ever since. Best guess I ever made with a car. It was mentioned on another string on this site but I don't remember where I saw it. Good luck.