MB Vito 112CDI Dashboard Fault

Just bought a 2000 Mercedes Vito 112 CDI Van 123k miles (5 seater with
window side doors) - with the cold weather we have had over the last 2
months, the dashboard has developed a fault (only when cold) the rev
counter and speedo dials move up and down in time with the dash lights
flashing and the clock has reset - took to MB dealer who could not
replicate the fault even by discharging the battery and they say it is
the dash but this will cost about £600 (claimed not to have seen this
before but there was another Vito owner in with a similar faulty!) -
this happened to me last week when de-icing the van and I couldn't get
the rear heated window to work. When the dash stopped flashing the
heated screen then worked. I don't understand how the faulty dash could
affect the rear screen. We also get the central locking to unlock when
driving randomly. Does anyone have any ideas before I get the dash done
and then find it is not the dash that is faulty and a waste of money.

Thanks in advance