X on Dell Latitide 'smudge'

[Erik van der Meulen <e.van.der.meulen@avondel.nl>]

Dear all, I run Potato (2.2.17) on a Dell Latitude CPi. For most I am
very satisfied, but there is one issue which slowly starts to irritate
me:

If I move large windows (which take up nearly all the screen area)
around, I observe a kind of 'smearing' of pixels along the outer areas
of those windows. I is not fatal and eventually the smear gets replaced
by 'aligned' pixels, but is looks bad. I think it happens with all
large windows, but I observe it mainly in Mozilla and Win4Lin (because
they usually run 'large'.
I have lived with this behaviour for about half a year now, and it has
survived upgrades of Gnome and various window managers, so I suspect it
so be the X configuration.

If anyone has a clue where to look and tweak, or an example XF86Config
for a comparable configuration, I would be more than happy.

Thanks a lot.

--
  Erik van der Meulen <e.van.der.meulen@avondel.nl>