My 2004 Pontiac Grand Prix seems to only start during certain hours of the day, what gives?

I have a 2004 Pontiac Grand Prix GT which has been giving me problems for
the past two months. When I crank it up first thing in the morning, it starts
right up. Then throughout the rest of the day, I have to give it a little gas
every now and then to get it going. If if don't give it gas the engine struggles
to get going, but eventually does. However, it's usually hit or miss. So
instead, I always press on the gas to make sure to that it does start up.
Night time driving is a different story. It seems like between the hours of 9pm  
and later  (I usually crank it up around 7am) it won't crank up at all, no
matter how much i press on the gas or how much the car had been driven
during the day. Could there really be something preventing it from being
driven during certain hours of the night? The engine has also shutoff of me
twice while in traffic, and will occasionally sputter from time to time until i
tap on the gas.  I had the the codes scan and i got these: U1000 AND
U10096. Any help will be greatly appreciated.