Your refresh rate is probably too low.
Right-click desktop, Properties, Settings, Advanced, Monitor
There's a little drop-down list. If you can't select anything higher than 60Hz it could be because your screen resolution is too high (drop it back a notch and try again) or your monitor drivers are crap (reinstall, update, if you're not using drivers it shouldn't be this)
Or something. Could just be a bust monitor I guess...
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