you still got to suspect the battery/charging system. it sounds as if the only problem is engine turning over. and that is a battery or storage issue. you may not be giving enough charge to the battery when running so check your altenator for output make sure of the age of the battery. if it's 3 years old well maybe you need a new one even if it's a 72 month.
the voltage reg is on the back side of the altn. wouldn't be out of line to do that anyway. appx 15yrs? they sell here for 18$
i believe auto zone can check your charging system. cold effects the turning of the engine both on the battery CCA and the cold oil etc.
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