This morning it was very cold (~15 degrees) and when I started our 95 Legacy Wagon with 65K miles it started right up but the AT Light was flashing. When I shifted into reverse it died right away. I restarted and the light flashed again (about 5 or 6 times I think) and again the car ran fine until I shifted into gear when it started to shake and die.

 

I left if for about an hour, then checked all the fluids, and when I restarted it then light didn't flash and it shifts into gear without dying. The snow is starting to accumulate so I didn't want to try driving it away for fear of it dying again when I was far from home.

 

Any thoughts? Seems like a sensor of some sort but I have no idea where to start.

You might want to just let the car warm-up for a little bit so that the fluid can warm up. After it has reached a good temp you might want to shut the car off for a minute and then restart and try shifting. Perhaps it doesn't like the cold. Maybe a block heater might be in your best interest.

Now I have no idea if this is true, or even true of the 4eat transmission in Subarus, but I was told that when it is especially cold you should shift an automatic transmission into neutral while warming it up. This allows the fluid to circulate more than when it is in park. If nothing else, it shouldn't hurt the transmission to do so.