Why wont my mouse, keyboard, and monitor work

Whats up guys. A few weeks ago I noticed that my monitor, keyboard, and mouse would not turn on (be recognized) by my computer when I turned it on. I did some research and saw that it could possibly be the motherboard. So I got a used MOB from my cousin and put it in today. Now when I turn it on the keyboard will sometimes work and the mouse will sometimes work. The weird thing is they do not turn on together. If I power it one time the keyboard will work, and if I turn it off and turn it on again then the mouse will work but the keyboard wont. At the same time my computer will not recognize the monitor at all. Any answers fellas?