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Many of mine change in brightness as they warm up but it is gradual and stops
after a few minutes.

If it happens all at once, then I would suspect a high voltage part in the
z-axis DC restorer like a diode or capacitor has marginal leakage and may be
close to failing. I think it would be easier to just do a general replacement
of those high voltage parts than trying to find which one is causing the
problem.

Before doing that I would test with a different timebase and do the z-axis
calibration procedures. If you want to make a measurement, check the low
voltage DC level at the output of the z-axis amplifier with a voltmeter and see
if it shifts when the brightness changes all at once.

I would not use freeze spray on the z-axis circuits because of condensation and
high voltage.

Another thing I might consider after the above tests is removing the z-axis
board and cleaning it thoroughly but if I did, then I would change the high
voltage diodes and capacitors anyway.