I have the Canon Speedlite 580EXII for 3-4 years now, and I have used it with a 450D and more recently with a 60D.
With both bodies, there was a very weird behavior where ETTL or even TTL will suddenly fail, and the output of the flash will be totally uncontrolled, it can be heavily under/overexposed.
It appears to be random and I have tried restarting the flash, didn't work.
I tried refitting the flash unit after cleaning the contacts with rubbing alcohol, sometimes it work sometimes it doesn't.
In the end I found out that it was not the dirt/dust, it was the connector. When I hold my flash, I can nudge it very slightly to the left or right, this is very minor, the distance it can be rotated is less than 0.5mm.
Sometimes nudging it to the left/right will re-enable the ETTL. Sometimes it will fall out of ETTL and TTL altogether.
Another symptoms that I find puzzling is that my flash seem to be skipping quite a lot more than it used to. On a fresh pack of fully charged battery, I will get roughly 2 to 3 fps (for the flash) even at f/1.4 ISO 400.
I have used the flash before and I know it is capable of doing up to 10fps depending on the battery, aperture, distance and ISO. With my 60D doing around 5fps, 80% of the time I will get 1 or 2 "dark" shots where the flash did not fire.
The 580 EX being the most powerful flash Canon has to offer, I am wonder what is wrong with my flash unit?
I am not a pro, so I don't use my flash like a workhorse every single day. I use it only when I can carry the extra weight and it is suitable to use a flash. I honestly do not think that my flash is already "worn out", and I think there is something wrong with it, and hopefully can be repaired.
Does anyone has a similar experience with their 580 EX II? Any advice or insight to why this might be happening?