Monitor Screen Shakes/Vibrates

Hi
I am using an external TV tuner card Frontech-0614. I have SAMSUNG 17″ LCD monitor. I am also using a VGA splitter/switch to switch between my PC and TV. Picture/image quality is perfect in the monitor when using PC. However if I switch to TV image starts shaking/vibrating. TV can be viewed but it is annoying. If I directly connect my monitor and TV tuner card (without VGA switch in between) picture is perfect. Any clue why is this happening and how to get read of it.
Thanks & Regards
Purnendu