Could it be power line transients? I had systems running on a 24VAC
external transformer that had 400V bridge rectifiers in them. The
rectifiers would occasionally fail. These systems were used in movie
theaters. I tried putting  a scope between earth ground (AC safety ground)
and one side of the secondary, then fired the xenon lamp in an adjacent
projector. I saw a spike higher than 300V. I can imagine higher spikes
blowing out the rectifiers. I believe the spike was capacitively coupled
from the primary to secondary of the transformer. If some part of the
secondary had been grounded (like perhaps the center tap if we wanted a
bipolar supply), the spike would not have gotten into the rectifier. But,
we needed a single ended 30V supply, so no part of the secondary could be
directly grounded. On this product, we moved to an external switching DC
supply and have seen no issues. In your case, it might be interesting to
try to protect the bridge rectifier with MOVs.

Good luck!

Harold


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