Hi John... That is an unusual one.  I know this sounds silly, but have you 
monitored the line voltage?   I get (In Bothell Wa, that is)  A four to six 
volt change at just about those times every single day.  Probably coincides 
with some compensation network in the power grid.  My stuff is unaffected as 
it is very highly regulated.  Good Luck.  Mike.
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