I also have a 5D and 580EXII. I have had similar problem. Mine appears to be the foot. It does not clamp down tight enough so as the weight shifts forward you loose connection with some of the connection points. If I hold the flash by hand and pull it back it will fire OK. I have been thinking of replacing the foot. I got a quote for a replacement from http://www.flashzebra.com/ordering.shtml#domestic but have not purchased yet. The quote was $47 + shipping.


Mark