I recently picked up a Sony Trinitron KV-30HS420 for use in our living room. I've had an issue with it that seems to be occuring more frequently as time passes. When the TV is first turned on, the picture kind of shakes, or bounces, up and down. Sometimes it only does it for a couple seconds, sometimes it can last as long as 10 or 20 seconds. After that, it usually stops and doesn't do it again as long as the TV remains turned on. It does it with our cable as well as the DVD player that's hooked up to it. Any idea what could be causing this and what I might be able to do to fix it?