T61/X3100/dock: DVI only works if VGA is connected

[tlc]

My T61 with X3100 running Kubuntu Feisty has been driving my 1920x1200 display via the Adv. Mini Dock DVI for months.

But when I moved the dock and LCD to a different location, I had trouble getting it to work again (even though I had no sw or config changes). The monitor was displaying a "signal out of range message". I rebooted a few times with the same result.

The only difference from the earlier set up was that I had had a VGA cable from the dock to the same LCD as well as the DVI (because of some initial issues getting video with the dock).

I added the VGA cable and the DVI now works!

Can anyone guess why?

I'd attach my xorg.conf and good and bad Xorg log files, but I don't see a mechanism...

tlc