You say “every so often”, how often is that? If your pump is out flowing your drain, I would expect every so often to be every few minutes. Sounds like your pump is pushing water back the the display faster than the drains can return it to the sump. Whether this is your particular problem or not, I’d still install a valve in the return line. There are many reasons why you would want to. Closing it for maintenance or emergencies is a nice option to have as well. I think it will fix your problem.