Thanks for the reply and it was one of the first things I'd checked-
Actually it's an integrated Intel G45 board and the overscanning is still set to 0 -
My assumption on what's happening (and I'd be happy to be proved incorrect

)
Since tvs normally have a bit of overscan built in to accommodate CE devices, we normally have to go out of our way to disable it - I believe it's called "just scan" on this model. As a 60hz signal, the tv believes what we're telling it and everything is fine. However, once we feed it a 24hz signal, it figures we're clueless since we're not sending it a "computer" refresh rate and "corrects" things for us. (i.e. assumes we've connected a CE device and turns the overscan back on)
I can make the image fit by setting the 2 sliders to 65 (horizontal + vertical), but I get the sneaking suspicion I'm doing the old "double adjust" thing. (Instead of sending a 1920x1080 signal and having it displayed as such, the tv is scaling the image and then I have to turn around and re-scale the signal so that it fits-so that I'm actually running something like 1900x1062 - like we used to have to do with a CRT RPTV)
Again - VERY happy if I'm wrong here and it's a simple matter of running 1920x1080 @ 24hz and then setting the scaling sliders to 65 - it just looks a bit blurry and doesn't feel right.