You've got an instrument cluster earth (ground) not making good contact with the body.
Use a short, thick wire to a self-tapper through a nice clean bit of steel bodywork (spray some underbody wax on the joint afterwards).
This will be much easier than trying to identify an old, corroded connection.
It's also worth eliminating the Hazard switch, as this sounds like it may be giving intermittant problems. Just pull the switch out and short across the terminals.
'73 Jensen Interceptor (Resting)
Giant Full Sus Mountain Bike