It's always the TV.
As the plasma is an older model with no adjustment for overscan, you have to play around with the resolutions to shrink it a bit to get all of the image on the screen.
Hence the need for a custom resolution.
No it is not always the TV.
I myself had an HTPC with a slightly older graphics card. On my full-hd plasma, with a clean install of videodrivers, the Ati graphics drivers by default activated overscan when it thought it was connected to a TV. And that setting was kind of hidden too. It took me a while also to find that out.
Any plasma/lcd tv has a fixed resolution display, and refresh rates it can accommodate.
So if you can find a table with those resolutions and refresh rates, certainly with contemporary graphics card it should be possible to set exactly that resolution an refresh rate.
Still no guarantees, since technology will always be a b****, but a good stating point would be to get the facts on these resolutions and refresh rates.