After doing a clean install of El Capitan on my iMac, about every third or fourth time I try to type in my password to unlock the screen from sleep, the password box will just start filling up with the dots that hide your actual password. Delete does not remove them. Also on trying to go to restart or shutdown, it is impossible to even type in the user name (The first letter I type takes some time to show, then often repeats). So the only way to reboot is a hard shutdown. On start up everything is fine again for a while. No problem like this ever on Yosemite, & the keyboard otherwise works fine (The whole rig is a refurb from Apple & only 4 Months old). Any thoughts what is wrong here?