2008 Nissa Sentra Will Crank and Start, but Goes Dead.

So I've bought a 2008 Nissan Sentra with 102K miles on it. I noticed that
every morning and any time it sits for a few hours, the car would start, but I
would go dead. However, every time this happened, I could try again and it
would start and run fine and as long as the engine was hot, there would be
now problem. In attempt to alleviate this problem, I took my car to AutoZone
and had them run a scan on the check engine light. The codes were for a
Camshaft position sensor(P0340) and a Throttle/Pedal Position
Sensor/Switch(P2122). So I started with the Camshaft sensor and changed it
out. At first the car started no problem (Engine was still warm), but after the
car cooled off, I almost never got the car to crank. I finally got it going and
took it back to AutoZone and had them run the service engine scan. This
time, the only code shown was the camshaft sensor scan, which I just
changed. Now, I can't even get my car to start. It cranks well and starts, but
immediately goes dead. Please help!