Hi Ruptor!
So, now we are getting someplace
To help you interpret Minerva's RAM failure messages, there is a simple Basic utility that came part of the Minerva software 'distribution' - of course, to run it, you'll need a working QL or emulator to-hand...
In short, however, the first long-word of hex digits is the value that Minerva thought it had written to the RAM, the second what it read back (should have matched line-1), the third, the memory address that showed this failure. '20000' in Hex is the base of RAM in the QL (at 128KB in the overall memory map - where the video-display is mapped.)
The actual values read-back can, in some lucky circumstances, help identify which DRAM IC(s) is/are at fault. The Minerva RAMFAIL_bas utility shows you this graphically. However, it only really helps if the error-bits in the values show some consistency between reboots, which is sounds like they don't...
Given the symptoms described so far, we can conclude that the ROM is being addressed correctly, CPU being clocked and running the ROM code, but that access to memory is being hosed...
That can be caused by:
A. A faulty bus-transceiver (the LS245)
B. One or both faulty Address multiplexors (LS257)
C. One or more DRAM IC failures
D. Shorts (or open-cct) on one or more address or data lines - probably 'inside' the data-bus managed by the ZX8301 ULA.
E. A faulty or on-the-edge ZX8301 ULA itself.
None of the above items would necessarily affect the ROM being accessed, as that lives on the CPU-side of the data-bus, and up until the RAM test, the only one taking any notice of the state and contents of the DRAM is the 8301 whilst it (tries to) builds its video-display.
We can also add another possible suspect - the EPROM itself. If its outputs fail to go tri-state when not being accessed (via ROMOE generated by the 8301), this will interfere with the data-lines that should be carrying the ram-test values back and forth from the DRAM during the test. Again, it wouldn't necessarily show itself until this stage during the boot-up sequence.
Keep us posted - we all love a QL-hardware mystery on a Friday night!