Hum,
Are you running anything like MODEPRO, check your WBSTARTUP drawer...(and startup-sequence & userSS)
It might sound weird, but try REMOVING aLL screen modes, and add (new ones)them back one at a time into devs: drawer
(and you know that if you hold down the two mouse buttons at start-up you can choose, er, options...)
It could be that the monitor is getting too old and losing the frequency...
" The thing about a 1084-- it's got lifeless screens, black screens, like a doll's eyes. When it comes at you it doesn't seem to be livin'... Until it bites you, and those black screens roll over white..."