Well your video drivers sound ok.. but there are other modules (ie. drivers) that *may* need to be installed. I'm not sure which ones, just saying that this is prol what is needed.
This is because you are feeding video that is *normally* only required to work on a smaller, computer monitors. Now you are trying to tell it to show properly, on a larger monitor (tv screen)... sooo, because of this special thing you are trying, it will almost definitely require extra modules.
Prol something HD related.
I don't know which ones because I have never had your exact situation.