CPU Bootup Problem with Pentland MPV930

I am experiencing failure to boot of my VME system whenever I am using
a set of Pentland MPV930 DIO boards (older than 2000).

My VME system consists of :
Slot 1: Motorola 2700 CPU
Slot 2: Motorola 2700 CPU, 2nd CPU boots over backplane, A32
Slot 3+4 : 2 slot VME SCSI Disk Drive
Slot 5: Pentland MPV-912, #1 (A16:0x7000)
Slot 6: Pentland MPV-912, #1 (A16:0x7080)
Slot 7: Pentland MPV-955, (A24: 0xF0000)
Slot 8: Pentland MPV-930, (A16:0x1000)
Slot 9: Pentland MPV-930, (A16:0x2000)
Slot 10: Pentland MPV-930, (A16:0x3000)
Slot 11: Pentland MPV-930, (A16:0x4000)
Slot 12: Pentland MPV-930, (A16:0x5000)
Slot 13: Pentland MPV-930, (A16:0x6000)

The system runs on VxWorks with a custom S/W.

Whenever I use a set of older (than year 2000) MPV930 boards in slot 8
thru 12, CPUb in Slot 2 hangs up during booting up, and ultimatley
never boots up the system. But replacing slot8 thru 12 with newer
(than 2000) MPV930 boards successfully boots up the system.

In an attempt to isolate this problem, the system was by successivley
tested as follows and the results noted:
(1) using less than 5 DIO boards (old and new) resulted with the
bootup problem not repeatable,
(2) Using 5 DIO boards (old and new) the problem is intermittent.
(3) Using 6 dio boards (old and new) the problem is repeatable.

I am looking for someone who may have a similar problem...and some
suggestion on what could be happening.