What lens are you shooting when you are having these focus issues? The 7D has a rather advanced AF system for a cheap DSLR and usually doesn't struggle to focus however if you are shooting low end lenses that stop down to f4.0 or f5.6 at full zoom this will kill your AF. The AF system needs both contrast and available light so when your lens stops down to f5.6 from say f3.5 your body is basically trying to focus in the dark and it will struggle.Seems the auto focus is more related to background than cold. I have a 7D and it is not real fond of red against green on water. They all work on grayscale and some cameras do a better job of handling it than others, but they all have little quirks about color. Seems to me if you are shooting anything other than sunny days you may have that problem.
Mine seems to throw a fit on a hazy overcast day when it acts up, hot or cold. Take it outside in the snow from the house on a really gloomy day and try focusing on a like colors in the foreground and background and it just hunts from infinity to minimum focus, my 60D does the same thing. Just not enough contrast to focus on.