Generally when this problem occurs when it is cold it is because condensation can form in the gas tank when the tank is not full. When this happens the condensation (water) can get into the fuel pump and freeze in colder temps, but that would have to be under 32 degrees obviously. I had this problem last winter, and I solved it by buying an additive that is made by Heet that takes the water out of your gas tank during cold weather.
It is odd though that you would have problems getting enough power for it to start that far above freezing temps...