Alternator light came on and the speed-o, temp gauge and rev counter stopped working?

Hello,

It does sound like a faulty wiring in the charging system which is intermittent.
You will have to check the main battery wires for any corrosions or damages.
You will have to trace the wires and see where the issue is. Check the ground wire. Make sure it is connected on the chassis.
You can also check various wirings by using a multimeter.

You might find this article helpful:

https://www.2carpros.com/articles/how-to-check-wiring

Please let me know of any questions.

Thank you.