My AMD powered Nettop PC display settings for 1080p dont give me a 50Hz refresh rate options of only 23.98Hz, 24Hz & 30Hz. When I had the same problems with a similar Intel box the solution was found on an Openelec web page which involved creating an xorg.conf file in the .config directory. I think there were sample Intel, Nvidia and AMD files but the page has now been deleted and Google hasn't found me any useful solutions. I seem to remember its a problem with the particular TV not correctly informing what modes it can do!
Chris