Could you have a very marginal leak over a couple of months leading to
intermittant problems ?

I kind of doubt it, at least not at any level of probability that
matters.

I would expect any leak to manifest itself as total failure in hours at the
most.

Agreed if it's a real leak. I suppose it's possible for there to be
some sort of internal contamination that manifests itself usage.

If you have the flashlamp, you could test for ionization.

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