DRAM 128K system has stopped working

[MbeeTech]

Hey Folks,
My DRAM 128K has stopped working. Symptoms are if you turn it on it doesn't even get a video synch. No beep from the speaker. Holding down M on boot doesn't get you into monitor. Holding M and hitting RESET sometimes gets you into monitor but none of the keys respond. Just hitting RESET gets either a garbled screen or a solid cursor. 
I've checked voltages (5.02v on the 5V rail) and the EPROM. (Looked a bit dodge so I re-burned a BN55 ROM image onto the BOOT ROM)
Before I break out the 'scope and the logic probe and start probing around, can anyone suggest where I should start my diagnosis?

Cheers and thanks in advance!

(Of course it failed as I was getting ready to take it to a retro computer meet)

Chesh

First thing to do, as it sometimes boots to a cursor or monitor (by the way if you can get into monitor, it means the M key is detected at some point), is to 
take the coreboard out and 'clean' the 25 way pin strips with a fine file or emery board. Insert & remove the coreboard from the baseboard a few times after this is done
and see if that makes a difference.  I wouldn't bother replacing the boot rom.