DS5300 problem

[blstuart at bellsouth.net]

A couple of weeks ago I asked about some memory issues I was
having with a recently purchased 5305.  Some feedback I got
was very helpful in leading me to invest in some memory specifically
advertised as for this machine.  To make a slightly long story
slightly shorter, things are better with this memory, but I've
still got some problems.  I'm still getting an error on the
LCD display that reads:

P0 91 FAIL MEM3*

which is particularly odd since I don't have any memory in slot 3
and I can't find any reference to test 91 in the service manual.
If I try to run with both processors, it just hangs on "Starting CPU 1".
If I run with just one processor, a serial console prints out the
message:

Console entry context is not valid - Reset the system

At this point, I have a few single letter commands available to
single step and continue execution.  Continuing leads to another
error where it complains about memory not being contiguous and
then purports to size the memory reporting that I have 32MB in
slot 0 when the DIMMs are 128MB each.

My suspicion is that one or both of the memory riser boards have
a problem.  The second choice is that the motherboard is bad.
I've seen on the net some report that this error can come from
a bad version of the eeprom code, but I'm not even getting far
enough to find out what version I have and to initiate an update.
The last case would be that I have some fault that is the same
on both of my CPU boards.

Has anyone seen this sort of thing before?  Is there any additional
troubleshooting information available outside the Service Guide?
Are the schematics out there anywhere?  (I know, that last one's
just a throwback to the days when we expected to get the full
engineering drawings from DEC.)  I'm really primarily looking
for some way to identify whether the problem is the riser(s)
or the motherboard before I drop any more money into it.

Thanks in advance,
Brian L. Stuart