Thinkpad T40 display keeps going off (dark)

My office Thinkpad T40 has developed this problem recently. The display would suddenly go dark and only way I can bring it back is either by suspending and waking it up, or by doing the Fn-F3 (which normally darkens or turns off the display) and then hitting some key. It stays on for sometime and then it goes dark again. Can’t figure out if it is a software or a hardware issue. Any help is very much appreciated. I am running XP.

thanks
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