This is usually a problem of broken solder joints in the Pushbutton Controller. The circuit boards are arranged in an "H" pattern and they develop cracks in the solder where they meet.
Here is a thread talking about the problem. (posts 37 & 38 have pics)
DIY page. Introduction to Resoldering Climate Control Unit
If your not handy with a soldering iron, and don't know someone who is, an alternative is one of the rebuilt ProgRama units.
As far as interchangeable, I'm not sure as there appears to be different part #'s for the W123 & W126.