I agree with Bobinovitch, try checking fans and graphics card, possibly even the monitor cable.
Is it a conventional (glass tube) CRT monitor or flat LCD/TFT type ?
Also does it fade, or does it go dark all of a sudden ?
When it goes dark, does it go completely black (possibly even power-save LED on monitor coming on) or does it change tint/colour?
Freezing is quite often an overheating problem (not always, but it's one of the first things to check), switching itself off is definitely usually a overheating problem and shouldn't be ignored.
Good luck
Lives of great men all remind us, we can make our lives sublime, and, departing, leave behind us, footprints on the sands of time.
Henry Wadsworth Longfellow
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