Please help, very frustrating screen issue with my Mac

Please help, very frustrating screen issue with my Mac.

This problem has been with me now for several months.

My iMac will not

a) start the screensaver via timer (only via hot corner, or 'test')

OR

b) switch off the screen after a certain time has elapsed (despite setting in preferences

I was hoping it was a bug that would be fixed by an update. I am fully up to date on Snow Leopard.

This is a major PITA for me as

1) It wastes electricity and energy

2) It is an iMac with a built in screen. The rest of the computer works as well as the day I bought it, so I take looking after my screen very seriously. Damaging/failing it would be somewhat of a disaster.

I've (at least tried) to remove all 'non standard' screensavers.

I could live without the screensaver working (though clearly absurd for my otherwise perfect computer) - but I must get the screen-switch off to work. I have set it for 5mins inactivity.

Please, if anyone has any ideas, recommendations etc, I would be most grateful.

Chris