Rob Nelson in issue RC192 mentioned problems with a slipping clutch in negative temperatures. I had a similar problem with my 1982 Yamaha TT250 in cold weather. I switched from my summer bike, a 1983 Yamaha XT550 when the snow started sticking to the roads.
Pulling in the clutch lever to shift gear, the lever would stay in! I discovered the cable lube I was using would turn to grease in freezing temperatures and not allow the clutch to close. I flushed the cable with diesel and the problem disappeared.