Why's my cursor skipping around, causing errors, esp mail & webforms?

I have a late 2018 Macbook Air. Often while typing, especially email and webforms, but also non-interactive formats like letters, etc., the cursor goes off, characters appear in the wrong words, a whole paragraph will be erased, etc. It’s really annoying to have to gohis [see?] back and replace words or whole paragraphs.

The weird . thing is, this problem plagued earlier Macs I’ve owned, back to Leopard OS at least. I wonder if there’s a bug in my software (that no one can detect) that gets copied from one Mac to the next when I transfer files and folders. No one I’ve spoken with has a clue, not even our local Mac repair shop. Any ideas? Thanks, all! Bob