Around 2am I noticed the voltage in my house changing. The only reason I noticed was because my bedside fan kept speeding up and slowing down. Usually my house has around 122v, so I got a meter and discovered that one supply leg was at 130v and the other was at 113v. The bizarre thing was that exactly every 2 minutes, the voltages would even out to around 120v. 2 minutes later, they would go back to being unbalanced. This repeated for about half an hour, then stopped with no apparent explanation. The voltage is usually rock solid. No appliances were running in the house and there was no inclement weather nearby. What could cause a situation like this?
Also check neutral-to-ground potential. Check if your house neutral bar is bonded to the ground bar. Unless you have other panels inside or outside, there should be close to zero potential at all points in the system. And if that all checks out while the main voltage is fluctuating, then the problem is somewhere outside your home or in the transformer.