Turning on the toaster causes the kitchen lights to dim and the microwave to slow and light to dim as well. AC switching on and off impacts all lighting and appliances. Anything added or removed from the load on the house impacts all other appliances. This is new. Could the AC being the largest system on the circuit be the issue?

It sounds like you have a poor connection somewhere in the panel, could even be your incoming main or ground. Since this involves 240 vac and potentially dangerous o[perations I'd suggest getting 3 electriucans to give you a bid and their thoughts on what is causing the problem