It's about 3 years old & this started about 5 months ago. It's a sort of self-inflicted standby lasting anywhere from 10 seconds to 5 minutes. This then recurs 7 or 8 times. Then after a few hours it happens again. It's nothing like the 'no signal' message I sometimes get after turning the monitor back on after a break. One other thing:- listening carefully there's a just audible clicking sound coming from it during these comas.
Check if cables are correctly connected and reset the options, if that doesn't help it probably had its time.
If the power light is going off as well then the power cable (or connection) may be faulty, or the internal power circuitry failing. Its probably not the video cable if you don't get the "no signal" messages.
If its not the cables, its possible that the CCFL (fluorescent tube) backlight or its control circuit is failing.
Could also be excessive dust causing overheating, which you may be able to mitigate by giving it a clean and a blast of compressed air through any vents on the back.
You should try it connected to another computer (or another graphics card) just to be sure its no the computer with the problem.