Typing gibberish on imac

At random times the typing goes crazy with strange symbols over letters. Sometimes it just types periods to end of line, as if I were holding down space bar which is not the case. If I shut down for a while it gets better when I start again, but then the problem will reappear unexpectedly. Using OS 10.13.6 on a four year old desktop. This is going on for a few months. Any idea what this is and how to fix it?