"Thomas G. Marshall"

The pot is getting old. Sometimes spraying contact cleaner in it
helps, but only if the resistance path of the pot isn't worn out.

If that were the case, then why would tapping the potentiometer help it? If
it's worn out, it's worn out. No?

Wearing of components can happen in different ways.
Maybe the wiper is (about to come) loose from its connection.

I suggest you try contact cleaner first.
If that doesn't help, replacement is the only alternative I see,
besides the pot being loose from its soldering joints in the PCB.
But that's not the case, according to Arny. Wave soldering is holy,
untoucheable and lasting for millennia to come.

--
Sander de Waal
" SOA of a KT88? Sufficient. "