Well, that is a new symptom on a notoriously bad design. It is a cable operated design, what is likely happening is that the either the cable that operates the cam or the cam itself is sticking some in cold weather, my bet is the cable as they can get moisture in them and frees and fail to operate. It may simply be out of adjustment just enough that with a stiff cable it won't come out of park. I would try adjusting it a little. If not replace it. It is #5 in the following picture
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It goes to the lever on top of the transmission. It can be adjusted by loosening the jam nut on the yoke at the end of the cable and turning them some, make suer though that you have plenty of thread left. It will require removal of the left hand fuel tank and may require raising the platform a couple of inches to gain access.
As a general note make absolutely positively sure that you come to a COMPLETE stop before placing it in park.
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