Mark is correct about the LCD in the 706, and I'm sure most other radios too. In my case, 12 below was enough to make the display unreadable for about 15 minutes. When this happens, it appears not to work although the greenish color is there. I didn't notice the drift, although I'm sure it was there, too.
As for high temps, if the display gets too hot (Mark should know about this one!) it turns fully black. A little air conditioning fixes the problem in a couple of minutes.