could this issue be just the simple fact that all the electronic components are nearly 30 years old
if it helps, i would suggest changing some of the capacitors in the electronic boards
these wild movements are typical of intermittent connections as rick suggests ....but it seems to me that that kind of overshoot relates to the charge pump dc averaging of the circuit that then drives the meter, a cap near the power supply area if it was low in value could well show similar sorts of issues as well as others in the circuit .....capacitors store energy ....well kind of ,,,, and if low in value could have drastic fluctuations and effects on the rest of the circuit
i would be looking at the connections at that multi pin connector from loom to instruments first
test the connections accross the ribon cable connections
then go inside and change with new capacitors that may have dried out ( common occurence in my industry ...tvs etc power supplies etc ) with same value
there are some prefered types that we have found consistently to be good performers ,,,look for Rubycon
as a brand name.
i have seen the same issue on a few of the earlier bikes ....,maybe the year of manufacture of the electronic boards more than year of production .....particually on seaniis 85 k amd albys 84 k ...but not sure of the electronics inside .....i have had no issues on my 86 model ....yet (but have solved the dieing speedo pretty well ...and only so far )
in this type of fault , it usually follows a batch of included production devices, rather than the design
does that help ?