The reason I mentioned A/C is because that is the one of the biggest surge/spike one runs into.
Are you CERTAIN the mains are clean and not the cause of the problem? The reason I ask, is because
a user here a few months back, had V problems that arose from the mains up the road in a different address.
If you have machine/s that draw a tremendous amount of current, they can cause a kick back of high
voltage. The processor boards get scrambled on some of my cams after lightning storms or power drops too
and need reboots from time to time.
An explanation:
I know this is a very basic concept, and has been described in many places, but I still can't 100% understand inductive kickback. simulate this circuit – Schematic created using CircuitLab ...
electronics.stackexchange.com
A good multi-meter with high/ low / average might find your transient voltage spikes.