In addition to all the fine advice Matt and Ike shared, I've found oxidized IC pins/sockets at times when going through my Series II. I've also found ICs with bent pins that "curled" up when someone pushed them in but were touching the socket just enough to get an intermittent connection. Granted, this is a rare problem but it's one I've seen a few times. When you pull a channel out, take the time to remove the IC (if they're socketed on a Series III), clean the pins/sockets, and reinstall. Even if this isn't the immediate problem it will go a long way to help you avoid more issues in the long run.
Cheers, -- Don
