Maybe this has something to do with the refresh rate your TFT is running at. Normally they are running at 60hz, as should your TV, too (as I think it is a pretty modern LCD TV, because of the DVI plug you mentioned)
But maybe, just maybe, your graphic card just sets itself to an other value, as soon as you plug in the TV (or at an other random occasion).
I'm saying this because most TFTs can handle for example 75hz, but sometimes they will just start to act strange, because they are designed to run at 60hz.
But that's just a guess from my side. Could be wrong about this