[5th_bike]

Didn't you have a rusty 99 GZ ?

In cases like that, for electric contacts that are getting unreliable with time: you disassemble them, if possible, and clean the crud and oxidation off the contact points, for instance with fine sandpaper.
The only alternative is to replace the switch.

Information on adjusting the valves can be found in the maintenance manual that can be downloaded from this forum. You will need a special tool to adjust the valves.
But you can check them with a regular feeler gauge.