On Saturday, 14 April 2018 19:29:43 UTC+1, The Natural Philosopher wrote:

The OP is convinced that slow voltage surges are the problem not
fast high-voltage transients. However, this is just an
assumption - there might be fast transients causing insulation
breakdown. Nobody actually knows.

Therefore it would make sense to try and achieve a reasonable
bandwidth if it can be done with little extra effort.

It might be something completely different such as corrosion of the
elements.

Its an interesting problem. I will have a rummage around for a
suitable transformer and make some measurements and report back
in a few days.

John