Simply_q
it'll sure tell you if there is a weak connection there.
it's certain there's a weak connection, or resistive connection somewhere , if not in one of the devices, then it has to be in the service.
I'm figuring the house is older than the equipment and felt to start from the begining ... at the service pole and follow it back to the outlets being used. Skipping around looking at this or that seldom plays out well.
I've seen far wierder stuff happen.