If not vibration stressing filaments, it's almost certainly
over-voltage. I take it the times you mention are ON-times.

One way to get intermittent over-voltage is an open/intermittent
neutral in your service drop. Aluminum cable/connector often involved.
If you suspect that at all, contact your utility NOW and ask them to
check. For background, what happens is that as load changes on one
service leg, especially swithching of inductive loads, the voltage on
either leg can spike up or down. Been there. (The lineman who fixed it
was a ringer for Pat Harrington.)

J