I own a 2005 Grand Prix GT - in the mornings the doors will not lock using the FOB or inside door switches. However after running the vehicle for a few hours the door locks work again. This only happens when trying to lock the doors, they always open. Weird.
It could be the door lock switches. When they are cold resistance in the circuit increases and as they get warmer it decreases. So, they are probably slowly failing, as a lot of electrical switches do, and aare in need of replacement.