Have you tried using newer/faster/different RAM. OS X gets pickier and pickier as the versions progress. 10.3 being alot worse than 10.2. OS 9 is usually fine with any old RAM, but X hammers it alot more. If the RAM isn't up to it, it could cause your issue, though it would usually be less consistent with timing and symptoms. You would get scrambled images as well as just a black screen. Worth a try if you have more RAM. Try PC133 if you have any.