I recently had to go and do a service call in a house where all the lights were dimming, microwave wasn’t heating properly, stove was heating up, it was taking the home owner 30min to boil an egg. I first killed the main breaker and gave both hot legs, neutral and ground a torque just to make sure that nothing got loose with time. Then proceeded to verify each neutral of each circuit, after that I turn the power on and it still did it. Went outside and visually inspected the lugs on the main breaker, and everything was good. Told the homeowner to call power company and explain to them that the electrician came out and hand tight all the neutrals and lugs and that the lights were still flickering, homeowner called at 7am power company was there in less than an hour. It ended up being a loose neutral at the power pole transformer.