With replacing heat sink compound, old fans and video cards. Since I only have what you write I'd try that.
My titantium G5 tower will go to a black screen after being powered on for less than 5 minutes. Sometimes there will be a repeating part of screen just in right corner.. about 1/4 inch worth. Also sometimes FAN will power on unusually loudly. Thought it was overheating... all fans work. No dust.
Think its video card? Or Logic board? Anyone?