As well as all the other points suggested .... I'd think it possible that
the trigger voltage is not adequate or only marginally so for whatever
reason.

*[Manual here]
<https://media.digikey.com/pdf/Data%20Sheets/Excelitas%20PDFs/MVS-7000_Seri=
es_Manual.pdf>
1*

Fig 4 page 12 show the circuity as you describe it.
At the tube has only 2 terminals the controller must be generating a high
voltage (many kV) ignition pulse.
Marginal operation could be due to poor ignition loop impedance, trigger
module issues, failing tube (apparently eliminated).

Loop impedance appears to depend on C1 and/or CR1 deep-ending on pulse
polarity.

A look at trigger waveform and voltage could be informative (and exciting).
The waveform across CR1 may be an OK proxy.


       Russell



1.
https://media.digikey.com/pdf/Data%20Sheets/Excelitas%20PDFs/MVS-7000_Serie=
s_Manual.pdf
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