> ?07 MCHK FR PAL

   Smells like hardware.

> ?06 MCHK DBL

   I know nothing, but, to me, that looks like a machine check while
trying to deal with a machine check.

   You could try the usual low-cost, first-resort techniques, like
cleaning, reseating cards (memory, PCI, ...), or giving the thing a good
thumping (when the disks are _not_ spinning).  Otherwise, I'd start
looking for some different old junk to use as a replacement.  (Or, as
someone else is sure to suggest, moving to an emulator on some less
obsolete hardware.)