Display settings & Monitor shutting down

My computer monitor has started randomly shutting down, the tower is still on. I took it in to be repaired last year for this same thing...they reset some display settings and it was fine. I don't remember what it was they reset though. Does anyone have any ideas?

I haven't installed anything new and the only thing I've done recently was to update Firefox and roboform.

I am using XP Home Edition
Intel® Celeron® 2.60 GHz Processor
40 GB HDD
256 MB DDR

 

Does it do this whilst you are actually doing things on the PC - like typing for instance or is it after maybe you've just been sitting reading a web-page for a few minutes?