Read the following as possible causes;
GtkRadiant often has the same
problem running on computers with multi-core CPU's that QeRadiant, the version that shipped with Doom 3, Quake 4 and Prey, has. If the above don't 'solve' the problem then it will more than likely be a 'true' driver issue, especially so running on Windows based PC's, or any machine running Intel based "HD" GFX chipsets/cards, that will need to be addressed directly in the graphics cards settings (or driver updates/rollbacks).
If using GtkRadiant 1.4 on Windows based computers it may be necessary to upgrade to GtkRadiant 1.5 (create an additional installation) if editing any game running idTech 4 (formally the Doom 3 engine).
