Display Switch-off at startup.

Most mornings during the first 10 minutes after startup - but sometimes several hours after, my display will switch off (with the computer still running). If I put the computer to sleep and straight out again, the display comes back on. Very occasionally, it will go strainght off again and I will need to do the same thing for the second time - either way it will then stay on all day.
I am running Windows 7 Ultimate 64bit SP1, and the power settings are set to High Performance with display off to 'never' and sleep to 3 hours, and I also have all the latest drivers installed.

System AMD Athlon II X3 440 Processor 3.00 GHz with 6.00 GB RAM

I'm flumoxed as to what to try next. Any suggestions please?

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