No input signal

I have an Apple G4 and after 6 years of faithful reliable service, just recently what has started happening is after switching on the computer it boots up normally but then after a few seconds the monitor screen goes blank and the message "No input signal" appears, then the screen switches back to normal, then goes blank again with the same message. The computer never crashes or freezes, but the screen goes blank back and forth. Then the screen settles into a strange fuzzy picture with the normal lettering ie File Menus etc becoming oversized!