Help: System lost DVI video on bootup (post) and linux console

I recently had a system crash, probably due to flakey power.

Anyway, since then my system is fine, but it won't work properly on DVI output.
Something strange happened and I can't diagnose the problem!

Originally I had a single cable from the video card (ATI Radeon 7500) to the
DVI connection on my monitor (Samsung 191T). It worked fine before, from boot
("post"), to Linux console mode, to X. I am running a Tyan S2469 motherboard
with 1.07 bios. Software is Linux, Debian "testing", XFree 4.3.0.

Now booting ("post") or linux console messages only show up on VGA. Behaviour
in X is unpredictable. On certain monitors (Viewsonic VG181) it detects the
DVI connection fine. That is, if both VGA and DVI are connected, it switches
to DVI as expected. But on other monitors (the Samsung 191T) it won't go into
DVI. When I looked at the logfile for X it reports two monitors, but both are
analog.

I've tried lots of stuff, changing the video card, the monitors, the cable,
etc. I also tried clearing CMOS and re-flashing the bios. Nothing helped.

Does anyone know what could cause this type of problem? Someone suggested
that the SMBus/I2C on the motherboard could be hosed. Is there a way to
diagnose this kind of problem? I do lot of hardware stuff, but this one has
me stumped!

Thanks,
Richard