You are saying it will change native resolution just fine?
When you attempt to OC it to 75hz it changes itself to 640x480?
I am not sure I can help, really. My current desktop monitor will OC to 75 even though it's a 60hz monitor, and it seems to work well for most desktop apps. It will not operate properly in most games at that setting.
Is 75hz @ (desired resolution) available as a supported mode, or are you forcing it in control software?