Sounds to me like the voltage stabiliser/regulator to the instruments might have failed - alternatively, check the earth from the gauge is good.
The stabiliser is usually tucked up behind the dash on the driver's side screwed to the ash frame under the scuttle. It looks like this:
and you can buy them from SVC for around a tenner:
https://www.s-v-c.co.uk/product/smiths-voltage-stabilister-negative-earth/Note that the stabiliser needs to be earthed as well.
