Just a follow-up:
Turns out that the problem was with the way the cards were installed in the
Q-Bus. The mixed Q22/CD-serpentine backplane doesn't allow you to insert a
Q-Bus card in the C/D slot of row 3. moving the three dual-height cards to
the left hand side of the backplane fixed the problem and allowed the system
to complete its tests and boot successfully.
Thanks once again