Transmission slipping in cold

[MaineCedes]

Hi all. I've owned this 123 for nearly a year. When I first got it in the cold weather, she would slip between second and third when it was car was cold. Once it warmed up, no flare whatsoever.
All through the summer months, she never slipped at all. But now that the cold mornings are starting to set in again (sub 50 degree nights) she is slipping again. And on two occasions this fall, I seem to have lost the transmission altogether. What has happened is it has slipped between second and third, but rather than take my foot off the gas as I normally would to let it slip into third, I have stayed on the gas for a brief moment. On those occasions, I have had to pull over to the shoulder and put it back in Park, then drive and start out again, and everything seems OK.

With this much info, would anyone care to give me a diagnosis. Fluid is always at the correct mark, but don't know much else about this transmission.
Thanks