Check the earth lead from the rocker cover to the coil bracket is in place, check all the bolts on the rear of the cluster are done up tight as these supply the electrical connection to the gauges. As diagnosis-dub says check the voltage stabiliser:
To test the voltage stabiliser, connect a voltmeter between the terminals shown (see illustration) with a 12 volt supply to the remaining terminal. A constant voltage of 10 volts must be registered. If the voltage is above 10.5 volts or below 9.5 volts renew the voltage stabiliser.
