samsung 409 hi def 60/50 hz problem

hi all i have a HTPC connected to my samsung via DVI to HDMI cable, when the pc boots up and the windows XP logo is displayed i get 1280*720p 60hz crystal clear !! But when the screen changes to 'welcome' page then to windows desktop the resolution drops to 1280*720p 50Hz!! this makes the desktop all bouncy and horrible. No matter what i do (using power strip nvidia settings) i cant get 60hz its always 50hz .

I think it must be the nvidia drivers, they recognise the tele as a samsung and its max refresh is 60 . Anyone else had this problem.