Hi Hank,
Yes this is a common and known problem. There are a few older threads here that will take you through what you (probably) need to do to cure it, together with one that even has pictures of a bad earth fix. It does involve opening your mixer up, so make sure that, you are confident enough, and back all your data up before you start. If I can find the threads I'll put a link on here. We have found that if you turn the master off to avoid damaging your speakers, then turn the offending channels trim encoder flat out a couple of times it can be a "quick" fix whilst onstage, but it's not good practice, and doesn't always clear the issue.
Quadcabby