Thanks everyone - it's my son's camera and he is away at school. He asked the professors and they couldn't figure it out. The local camera store suggested lowering his shutter speed - I've passed that along but it sounds more like a "hot shoe" issue the more you guys explain it to me - it will work and then not work which would make sense if the pin connection is not solid. I have told him to take it to the local camera shop near his school (he's schooling out of town) and see if they can tighten the "hot shoe" mount.
If that doesn't work, do any of you use a "non-Canon" flash on your Canon cameras? He's not in a position to be able to purchase the new 600 EX flash at this point but he needs a reliable flash for his photography program. Any insight you can offer would be greatly appreciated.