Definately don't rule out the power supply being faulty.
If there's excess ripple on the 5V supply (due to lack of DC
filtering), it may cause erratic operation.
So use an oscilloscope to check +5V and +12V supplies are +/- 5% and
ripple is less than 1%.
Also try a different CPU card, anything like an EC030 card or A3640 is
fine to test with. Make sure the two clock jumpers are set correctly
depending on what CPU board you're using.
If it is a memory problem, and you've removed the fast mem SIMMs, and
system still faults, the RAM control logic (Ramsey/Bridgette?) might
be faulty?
The system should still boot fine on only chip memory, and no fast mem
installed.
After that, it's anyone's guess.
Also check all surface mounted electrolytic capacitors for signs of
leakage or corrosion (soldered legs are black instead of silver), and
black residue on PCB. I've had a few of these lately which can cause
open circuit PCB tracks. The 22µF ones tend to be the worst for
leaking.
Anthony.