Look into the possibility that the dip tube(tube that brings the cold water to bottom of the tank, hasn’t disintgrated) It is usually accompanied by restricted flow. There is a class action lawsuit(ending this month by the way)that addresses this issue.When you use the hot water, it is hot until cold water starts filling in , now at the top, mixing with the water leaving for the house. If your WH was manufactured between 1993 and 97.They have a website just for it.