Can you show us a screenshot please ?. Overclocking a G-card too much can result in funny images or currupted desktops as you describe. Other than that make sure you have the latest Drivers installed and i would reinstall also doing the following :
Installing Graphic card driver efficiently!!! When you Update your graphics cards drivers theres ways that you should go about it. Follow these simple steps below.
1.Download and install ''Driver Cleaner'' Here:
2.Uninstall your Current Display drivers such as your ''ATI'' or ''Nvidia'' drivers.
3.Now run ''Driver cleaner'' and depending on the Drivers you have you will have to select either ''ATI'' or ''Nvidia'' from the drop down menu. Only select the main title of the driver in the drop down such as just ''ATI'' or ''Nvidia''
4.Once you have selected Your driver click on the ''Start'' button and this will now start the driver cleaning process. ( Do this twice to be sure )
5.Now reboot your system and Download the latest Nvidia or ATI and go ahead and install them.
Doing this avoids driver corruption and conflicts so you can now enjoy smooth gameplay.
However try 75HZ for that screen as your screen may not be able to cope with 60HZ