We get calls for this quite frequently, and 90% of the time it's where the poco splices to the mains coming into the service. A couple of simple questions usually will point me to a poco problem:
1. When does it happen? (eg. At dinner time when the stove is on, dishwasher is on drawing water from electric tank, and laundry is being done - all big loads)
2. What is affected? (more than one circuit, but all on same phase)
Check the poco splices at the house - from a distance you will often see that one leg has been overheated.
