|First, delete all cookies, history, temporary internet files, etc.|
Second, check your browser's add-ons to see if there is something there that you did not specifically add to the list, and remove it.
Third, try raising the security level of your browser from Medium to medium-high.
Fourth, Scan with Malwarebytes.
** Do the first two weekly, do the third only if needed, and do the fourth maybe once a month or so, or as needed.
You have to be a little bit crazy to keep you from going insane.