Resolution changing on its own

Hello,

I have a 2003 20" Cinema clear acrylic display hooked up to my MacPro and it's been working fine for the last year. Over the last couple of weeks the display resolution has been changing to a smaller resolution by itself. I'll be away from the computer and can come back to the screen resolution different from how I left it.
The change can last for a few days, then it will change back to the proper resolution (1600 x 1000). That can last for a few days, then change back again...!
It's very frustrating as I then need to scroll up and down, side to side to view the entire screen.
I've tried changing resolutions, colour calibrating, resetting the PRAM, loaded SwitchResX Preference pane software, run hardware diagnostics but nothing works.
Also - when the screen goes to sleep, it will come back momentarily in the correct resolution, but then change right away to the incorrect one.

Any suggestions where I can start diagnosing?

Thanks in advance,
Mitch

Message was edited by: Mitch M.