You could be getting a spike from whatever. A small breeze with dust can cause a a high voltage situation that the lightning suppression won't see. Any disruption of load can cause a transient spike, a flip of a breaker feeding a load, let's say, can cause a problem. The power side of your cards probably do not have any spike protection in them, might be something to look at.
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Could be worse, it could be me. :)
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