Its probably just a cold solder joint,if you are able try resoldering the display pins in the radio,if not send to a tech who can.My father's truck mileage odometer did that I resoldered the entire display and now you need to be blind not to see it,its super-bright.
If you only experience this on a wall-wart plug then you may need a new wall-wart plug.
Try it with batteries that are new(not radio shack batteries) and then the wall-wart then rreport back.
I opened up the radio and the back of the display is covered by a shield that is soldered to the
board. I didn't want risk taking the shield off. (too close to other surface mount stuff)
Looking underneath the shield the display leads are too close for my soldering iron.
Looks like I'm going to sell this one as a tech special. ($50)
If anyone is interested, let me know.