Troubleshooting Flickering Lights

My lights considerably flicker whenever a large load comes on in the house and it has been this way since we renovated five years ago. I am attempting to troubleshoot this now and have already probably waited longer than I should have. I did some reading and ultimately removed the cover of the main panel and tested across the two hot wires at the main panel. All circuits were active. The reading I received was 245 +/- 2V, meaning average of 245V but fluctuated between 243V and 247V. I then had my wife turn our whole-house air conditioning unit on and voltage dropped to 237 +/- 1V while AC was running. Is this a normal voltage drop when a large appliance is turned on or is this indicative of an issue?