Matrox Triplehead2Go Display Problem

We have a Matrox Triplehead2Go installed with an iMac running El Capitan. It's not bothered us before. We added a third output, tried to adjust the resolutions, and now the main iMac display is messed up. When we unplug the Triplehead USB, the main display returns to normal but we aren't able to adjust the Triplehead settings with it unplugged. As soon as we plug it back in, the screen turns to squares and slows down.

I've tried uninstalling the Triplehead and reinstalling it without any success.

If we boot into Safe Mode, the display is fine - even with the Triplehead plugged in.

My guess is that there are drivers somewhere that are overriding the main display settings when the Triplehead is plugged in - but I have no idea where to find them or what to do with them.

Thoughts?