I have always used the cables that either came with the switches or where recommended by the manufacturer. In the case of the OmniView that I just bought I am using their recommended cables.

I know it isn't caused by cable length or routing, if I connect unplug the cables from the switch and connect them together without the switch in place the ghosting is gone. The ghosting is the same whichever source PC signal I am viewing.

Like I said, I have seen this issue with every KVM box I have ever used. Some of them weren't "cheap" (just paid over $200 for this one) consumer ones.