I'm about ready to hire an exorcist but I'll try to give a technical diagnosis one more go.

 I have a dma2200 linksys media extender (latest firmware) that is connected to a quad-core, dual tuner, dedicated Media Center running Vista-64.

The network is all gigabit wired ethernet.

When the extender is connected I get the low-bandwidth/network issues incessantly on lots of channels with it most profound on HD channels. When I run network tuner and diagnostics it shows ample bandwidth.

In alternative solution, I have a less powerful, general purpose windows 7 PC with a single tuner. When I connect the same extender in the same location to the same cables I have zero network errors.

 Is the problem:

A. I need to configure the network card in some way on the Vista-64 machine to fix these problems?

B. Is the problem a vista-64 issue that would be fixed with win-7 upgrade?

C. Something else?

Network card or network driver issue

Try toggling flow control in the network settings.