When I was running Windows I bought a small utility which did wonders, called TVTool (http://www.tvtool.info/
). When I switched to Linux, I was certain I would have to live without all the goodies that TVTool allowed. But -- surprise, surprise
-- I found out about a project which offered more or less the same functionality, if maybe in a slightly less polished package. Have a look at http://sourceforge.net/projects/nv-tv-out/,
install it and play around with it, you might just find the right settings.