I have a 2-story, 2400 sq ft home that was built in 1987 with what appears to be 3/4" copper pipes for the water. When ever a toilet is flushed, the washer is filling up, or the shower is running, there is a serious drop in pressure. There is no filtration system and we are on city water. A check of the pressure shows it to be at 120 psi then drops to 20 psi if toilet is flushed, remaining there until it is full, and then the psi returns to the 120. Suggestions on where to start on this?