LCDs are notoriously bad at supporting multiple refresh rates... it may not be capable. You can try unclicking the box that says 'show only supported modes' or something like that, it will show you all available, but may not do you any good.
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Every time you think you've made it idiot proof, someone comes along and invents a better idiot.
?the end of our exploring, will be to arrive where we started, and know the place for the first time. - T.S. Eliot