Also under "Filter Keys" and "Sticky keys" - go into the additional settings area (might be called "Setup" ?) and uncheck the "Turn on when pressing X after Y seconds" settings in each area.
Are you on a laptop? I've seen folks accidentally brush their wrist on the track pad as they type and cause stuff like this to happen, even going so far as making the cursor jump to another spot on the page and screw up what they're typing. If that's the case, turn down the sensitivity on the track pad.
When this happens for me, I repetitively tap every key on the bottom row of the keyboard (Fn, Ctrl, Alt etc...) because I know on my laptop sometimes it get's stuck, or thinks a button is pressed. Works every time! :')