Check connections between door and door post, usually covered with a rubber boot, check connections for signs of corrosion, I use a spray contact cleaner on mine. then make sure all moving parts in the door are greased up, small parts wd40 can work well, you need to get rid of any moisture in the mechanism, that probably freeze in winter. But contacts first, in winter they would contract, and if they have a small amount of corrosion, could be going open cct in winter.
SD, March 2012