monitors tell the computer which resolutions they support.

sometimes.
most of the time, they should.

And then there are the graphics drivers.

Somewhere in the OS some piece of software is deciding that the monitor output cannot be run at 50Hz, this is beyond my control.

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50Hz refresh on a CRT would flicker terribly.
But on an LCD it might just be acceptable.
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You can run the TV ( svideo ) output at 50Hz.
But this isnt any good for you as I think you are using a high resolution output 1280x1024?