I have a Samsung 955DF monitor that's ~3 yrs old, and just out of
warranty.

After performing flawlessly until now, I've noticed that it drifts out
of focus, but only when it's displaying a lots of white.

For example, when viewing Web pages with white backgrounds, or
black-on-white MS-Word or Excel document, after about 5 minutes I
notice it loosing focus. After 20 minutes it's actually painful to
look at.

The odd thing is that this only occurs when there's large quantities
of white on the screen (i.e. backgrounds). I don't notice it with
other colors.

Also, it doesn't worsen just by virtue of how long the monitor has
been on - my initial thinking was that it was heat related (and easy
to troubleshoot with a can of freeze spray). It only starts happening
when the screen contains lots of white.

Thinking about it now, I'm wondering it this is really a focus
problem, or if the display is "blooming" over time.

At any rate, any ideas where to start looking for the problem?

Thanks