I upgraded my gfx card to an nVidia 7900GT, patched the bios cos it was a 512MB gfx card and have it running under OSX nicely....with one small issue.

When it boots initially the screen is slightly garbled/blocky in appearance.
If I move the monitor to the other DVI port once OSX is running it's fine. If I then reboot leaving the monitor connected where it is, it comes back garbled and I have to switch again.

It doesn't matter which DVI port it starts on, it always requires me to switch the connector to the other one once I get to the desktop to get proper GFX display.

I'm rather stumped by this.

Any clues ?