On the back side of the house (kitchen sink, bathroom and upstairs bathroom shower) water runs very slow and getting slower as time goes on. On the front side (upstairs hall bathroom and outside water source) water runs strong. What is causing this problem :(

Are your pipes galvanized, bet they are. Over time galvanized pipes can build up a significant coating of minerals reducing the pipe to next to nothing. If this is you case there isn't much to do short of replacing the pipes, I'd suggest PEX.