(In reply to Jean-Philippe Gari�py from comment #11)
> I'll try using another desktop environment tonight.

Not just another DE, but another OS, like a Knoppix DVD or other live media, to
help determine whether a software problem, or failing X300 RAM. If it's RAM,
maybe trying a lower resolution mode or display would tell that your real limit
has dropped to less than 2048. Was your X300 manufactured by MSI, or shipped
originally in a Dell PC?