9 times out of 10, it's the wall outlets themselves that is the issue. General construction habits are to save money by saving time, so commonly outlets are 'stabbed' not 'wrapped' by the wires. Over time, the tiny spring brass in the outlet gets soft, looses a lot of its contact with the wire and this'll affect available amperage in the outlet as resistance is built up. Over long periods, this'll heat up the outlet, making it worse, until it melts or a fire is started. No joke. It's real.
There's only one cure for this, have someone knowledgeable pull out all of the outlets on that circuit and wrap the wires around the screws instead if having them stabbed in the back. If any look suspicious, replace the outlet. Make sure all...