Satellite and Cable TV

I have used a pair of these Netgear Powerline Adapters in the past, hooked one up to xbox and the pair to the router in the room on the other side of the house with the computer. The connection was perfect and always worked out fine, now interruptions. I decided to buy another pair for the other side of the house to plug in to another computer for connection. Now there are two adapters plugging into the router in one room from same outlet, and the pairs are on the xbox and another computer, both on seperate sides of the house, the router in the middle. Now, if all four boxes are plugged in, the internet slows down to speeds that are unusable. Any recommendations on what I can do to fix this problem?