580 Sometimes Over Powers on it's Own?

I've had a problem with a 580 flash recently. Sometimes, it vastly overpowers the scene. This happens both in ETTL and M. Flash is on camera and it just massively over exposes; again, sometimes in spite of being in M. After a while, or sometimes turning on and off, it will reset itself and behave better (though sometimes, it continues even after turning on and off)

Has anyone every experiened this? Is there something I can do to more reliably reset it? Is this a job for Canon service? Or does this just happen from time to time?

Thanks.

This is usually caused by dirty contacts or because the flash isn't fully seated on the camera's hotshoe. If you're getting full dumps while it's in Manual, it may need repair.