I am guessing that monitor is a few years old. what you are describing is what happens when sony trinitrons start their downhill slide to eventuall complete failure. from my experience this starts when the monitor is about 3-4 years old.
if that is the case with yuors then I would suggest shopping for a new monitor.
Get a new monitor! Had the same problem with this same monitor at work. It started out the same way you describe, eventually the entire screen would turn red then cut off. It finally died.