Check out any printed circuit board for bad solder connections. We used to get the digital dashes working by resoldering the joints on the power amplifier. I thunk the labor charge was 1 hour total and success was easily over 90%, high enough where we did NOT charge if the repair failed to get the dash working.
Digital dashes for "Disco" era z cars havenot been available for a very long time and were very expensive.
Even going back to the 60s you could usually fix things by backing off and retightenting the speed nuts that held the circuit board to the cluster housing.
Voltage drop, voltage drop, viltage drop. Find out where the power gets to and you have found your bad connection, right after your last reading.
Problem is the boards are getting smaller. I use a 10x loupe to do mine.
regards
mech