There is a communication problem on the Bit Serial Data line (BSD) to the DME.
Could be a bad IBS, Oil sensor, alternator (voltage regulator) or shorted wiring.
Start by inspecting the BSD wiring to each of these.
If ok, unplug the IBS and see if the problem goes away - if not try the alternator. Oil sensor last as it gives you the visual display if communication is present with either of the others unplugged.