No shift above 2nd gear when hot

[stirfoo]

2005 Legacy auto will not shift into 3rd when it gets hot. And by that I mean the outside air temperature. That seems to be the only factor. During cooler months it shifts fine. In the summer, I can drive it a while and then its like 2nd disengages, and takes progressively longer to engage 3rd until it will not shift into 3rd at all. I have to manually keep it in 2nd to limp home. Manually shifting to 3rd does nothing. If I let the car cool for a couple hours, the process starts over again. The engine temp gauge always moves to the same position, right in the center. I've had it flushed/new filter. Had no effect at all.

 

Thanks!