Rev 4 mobo disk trashing mystery

[schmidtd <schmidtd@my-deja.com>]

I picked up a rev 4 B&H machine the other day.  Integer ROMs, raised
power key, dark green slots, serial 27xxx something with 7930 date.
It's temperamental as those machines tend to be - and I have some
keyswitch rebuilding to do as well.  One problem is truly vexing me:
it trashes track zero of any disk I feed it when booting.

I have tried different power supplies, different disk controllers,
different ROMs (I plugged in a set of Applesoft ROMs on a switchable
card), and even swapped out every chip from a known-good mobo.  Still
I get the same behavior: booting with the disk card in slot 6 causes
disks to lose their minds.  They become unbootable, uncatalogable, and
have to be reformatted (not just re-imaged) in order to be reused.

What on earth might be causing this?