I repair Mamiya RBs and have had similar problems with all types of cameras and shutters during cold weather. You should keep your camera under your coat till it is ready for the shot and stay out no more than a few minutes at most. If you have to keep it out for long periods, you'll experiance a plethora of problems ranging from dead or low batteries to frozen shutters n condensation build up. Battery dependant cameras suffer the most, manual cameras hardy notice the cold if serviced properly.

Anyway this is a first sign your camera needs a little TLC aka CLA... cleaning, lube and adjsuting. How old is the camera and when was it's last service?

Old lube in shutters and mechanisms' gets gunked up in extreme cold and can slow your shutter or also cause it to sporadically fire. Another problem is condensation build up in the camera which can freeze moving parts. Also the slightest amt of oil on iris or shutters blades will keep it from working. While you are servicing the camera, check the light seals too.