Do anyone use ADD2 DVI dual display configuration?

I bought one ADD2 DVI card for an old HP dc7700 and found the NetBSD GENERIC
kernel console fails to show a higher resolution on DVI.

- HW
Selected number of Intel built-in graphic north bridge, Q965 in my
case, allows ADD2 DVI
card for DVI output. It's a mere Silicon Image chip to feed DVI.

- Short summary
i915drkms0 pauses during VGA console mode or a while and then both
displays go dark.
When DVI display cable is not plugged in, video mode switches to a bit higher
resolution successfully, as good as so far, and everything works fine.

- Details
Just after power-on, dc7700 shows BIOS startup in VGA on both display,

agp0 at pchb0: i965-family chipset
agp0: detected 7676k stolen memory
agp0: aperture at 0xe0000000, size 0x20000000
i915drmkms0 at pci0 dev 2 function 0: Intel 82965Q Integrated Graphics
Device (rev. 0x02)
Intel 82965Q Integrated Graphics Device (miscellaneous display,
revision 0x02) at pci0 dev 2 function 1 not configured

The last line might imply the cause.  I can provide more diagnoses if requested.

Toru Nishimura / ALKYL Technology