Cause of Voltage Drop
You may have a lose connection at the main or in the meter. Check the areas above the branch breakers. Particularly at the main breaker the main bus below the breaker and the lugs at the meter. Look for signs of overheating or burnt insulation. Also if you have an infrared thermometer look for hot spots the drop could be coming from the internal parts of a breaker. Picture attached for an example.



It may not look as bad but it will in time. If you find the location you need to replace that piece of equipment asap.

Good Luck