A friend and fellow photographer mentioned that he was having the same problems with his camera and his brand new 580EXII and couldn't figure out why it was doing that. I finally found the cause and thankfully, the fix is really easy and inexpensive to do. If you are experiencing the same problem of random full power flash dumps, try this and see if it works for you.
***DISCLAIMER*** I make no guarantees about this repair and have no idea if it will void your warranty. I only know that this procedure was simple and fixed the problem for me.
All you will need to do this repair is a precision screwdriver set which you can find at Amazon for about 6 dollars.
1. Make sure your camera is off. Mount the flash on your camera's hotshoe and lock it down.
2. Very gently (and I mean VERY GENTLY), try rocking the flash from side to side and look at the camera's hotshoe to see if it flexes. If it does, then you have found the problem. Poor connection of the pins on the bottom of the flash to the hotshoe will impair the communication from camera to flash leading to misfires.
3. Take the flash off the camera.
4. Looking at the top of the hotshoe, take a small flathead screwdriver and gently slide it in and under the inner plate of the hotshoe. To remove the plate, slightly lift it up with the screwdriver and you should be able to slide it out easily (don't force it).
Remember which way the plate faces and which ends are bent up and which are bent down. You will need to put it back in the same way.
5. Once the inner plate is removed, you will see four tiny Phillips screws.
Take your smallest Phillips screwdriver and tighten each one until the hotshoe no longer wiggles around. Be careful not to over tighten and strip the screws.
6. Replace the inner plate. Be sure to put it back in the correct orientation.
7. Reattach your flash and test it. You're done!
Hope this was useful to some of you who were having the same problem.