I am sure there should be a voltage stabilizer
… Check behind the speedo and tach.
cgi.ebay.co.uk/Lotus-Elan-Triump … dZViewItem
This electromechanical type does go wrong from time to time. Check it is properly earthed. If it is no good replace it with a new one.
You can use better solid state ones - look on ebay.
cgi.ebay.co.uk/SOLID-STATE-VOLTA … dZViewItem
Also, try searching the archives here for more info.
best regards, iain