I played around with this again and it still crashes randomly. The idea is it watches the missions and makes a sound/flash/whatever when it encounters the "Complete!" text. Most of the time it works fine but every now and then it hangs up and crashes to desktop when a mission completes. I've just about gone through all my characters with the easy missions for testing
I'll look at it again but can't give anybody a mod that's certain to crash the client, even experimentally.