i did that too...except when i changed either the clock or core speed, it said, "Nvidia has to test your settings to make sure they are compatible" and it won't let you continue without doing that. So once you do it, it says, "These settings are not compatible with your system, try lowering the speed until it works"
problem is..i lowered the settings from what they were at, stock, and it still says lower.........whats wrong?!