do you also have brown outs and voltage drops?
if so, look for large loads that start / stop (200 hp air compressors, large welders) for example.
you should also look at the nearby industries, they can cause power spikes , voltage drops. if that is the case, you can have the power company force them to correct their power factor citing equipment damage. it's a pain when they get hit with a 100k penalty for power spikes and get charged based on that spike demand.
james