DRAM 128K system has stopped working

[CheshireNoir]

OK Folks,
I broke out the ROM Programmer, and then dug out my collection of 27128 ROMs left over from upgrading the PC85b back in the day. (The very same main board underneath the new DRAM core board I'm playing with now. The Core board is set aside for now. I'll probably look at upgrading the Personal Communicator with it if I can get this DRAM Core board working reliably)

What I have tried:
BN5443 - No change. Still doesn't seem to boot.
BN55 - I checksummed it and the ROM itself is fine
BASICA+B - It boots into basic fine! The keyboard works and I can type my "Hello World" 2 line BASIC program in.
(The BASICA+B is just microbee-pc85-eprom-a.bin and microbee-pc85-eprom-b.bin burned sequentially onto a 27C128)

What does it all mean??

BTW I also tried pulling the FDC controller chip in case it was a bad chip, but there's no change. Tired it with both the BN5443 and BN55 ROM.

The scope is on the bench, and I can get the risers out at the drop of a hat if need be :-)

Chesh