On Sat, 21 Apr 2018 00:31:21 -0700 (PDT),
wrote:
snip

I would suspect things around choke. Clean and reconnect the terminals. Sometimes cobweb or cockroach muck could cause change in voltage across the choke.

I did have a look in one of the two units (that are behaving
similarly) and it was as clean in there as when I installed it from
new (you don't live in a seedy restaurant in Thailand do you)? ;-)

As a total unit, once the plasma is struck the total energy is sum of choke drop and the plasma drop. Plasma energy is generally low, hence choke is the component taking most enrgy. Choke may develope intermittency due to failing insulation.failing insulation.

That would have to be on two independent units then? Not impossible of
course but at the same time after maybe 20 years?

Cheers, T i m