After working in Excell, Word or Powerpoint the keyboard will suddenly start displaying random letters....If letter "A" was working ok it will suddenly start displaying a c or d or e or some other letter. The unexpected letter is now key "A" and doesn't change until the program is shut down and restarted....This doesn't work every time. I've had to turn off the computer at times to get a reset back to normal. What's happening? How do I stop it?