Are you checking the voltage under load? Check both sides under load, preferably at the main to see which side of the main the problem is located. If one side goes down and the other side stays steady, the hot leg is bad. If the voltage on one side goes down and the other leg goes up, the neutral is bad.
From there you need to isolate the problem. If you check it at the main, you know whether it is "in" the house, or "outside" the house. Even better is to check it at the meter base if possible. That way you can tell the POCO you know it is on their side, or you know it is on the customer side.