My 580EX II flashes are not working consistently on camera, they are fine off camera (fired by infra red)? What could be causing this?

SOURCE: My snaps are underexposed with 350D camera and 580EX TTL

Set camera:

Single shot mode not servo/C modes
Focus sensor set for center
Make sure exp. comp is not set on -(minus) side.
Make sure your subject is locked on and not moving out of the autofocus (center) sensor.

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SOURCE: Canon 580ex "pops" in a baad way

It is very dangerous though... [referring to Alexander's solution]... I've been shocked more times than I care to really admit, and that cap can cause some serious damage. Be careful!

Lance.

SOURCE: 580EX speedlite not firing

Does the pilot lamp light up? If so can you fire it manually by pressing on it? If the batteries are low it may simply just be taking a while to charge, the camera will not wait for it to be ready to fire and it may seem to be not working. If your flash can be fired manually however, there may be a problem with the cameras shutter or the hotshoe. You can do a basic check by pressing the star button at the rear of the camera, you should get a pre-flash. If not, there is a problem with the cameras hotshoe which you will need addressed by a service centre. If you get a preflash you can next check the cameras shutter by setting the second curtain to sync with the flash. This can be done via the cameras "Custom Function" menu, c-Fn 15. Set the camera to TV mode and chose a 1 second exposure. Take a shot with the flash attached, if the shutter is working correctly, the flash should fire a pre-flash and then a main flash. If it doesnt, the switch inside the shutter is broken and the shutter needs to be replaced. This is a pretty labour intensive repair and you may consider an upgrade as more economical.

SOURCE: 580ex off camera shuts down after a minute or so of standby

No and no, the flash is by default programed to shut off after 1 minute .If you were using the flash on-camera, the flash would automatically power itself on when camera activity is detected but when the flash is detached, it doesn't wake back up.
To change this you have to turn off the auto shut off in the flash menu:
1. Power the flash on
2. Hold down the back light /C.Fn button until "Fn 00 0" appears on the screen
3. Rotate the selector wheel until you see "Fn 01 0"
4. Press your selector button down so that the 0 begins flashing
5. Rotate your selector wheel to change the 0 to a 1
6. Rotate the selector to the "Fn 14 0" position and set it to 1
7. Press your selector button to save this parameter
8. Press your Mode button to return to the main screen
Now your flash stays on as long as the flash switch is turned on ON, and as long as you have juice in the batteries.

Do not forget to turn off the flash after you are done.You can reverse it anytime, but you have to read the manual for that..

Same setup can be found in the manual

SOURCE: I mounted the Canon 580EX II to my Canon 5D Mark

Yes. If you were able to set a faster shutter speed, then you would not expose the entire frame and would have the shadow of either the first or second shutter curtain (or both) partially masking the frame.

At higher speeds, the shutter is never fully exposed: before the first shutter curtain has finished travelling across the frame, the second one has stated it's journey. All SLR's have this issue and on some older models you could only use a maximum 1/60th of a second.

In practice though, in dark conditions the "slow" shutter speed does not affect exposure as the true exposure will be determined by how much light the flash puts out, and it puts this light out in as little as 50 microseconds (50 millionths of a second) for a modern electronic flash bulb.

Faster shutter speeds can be used successfully, but only with flashes which operate in high speed mode. What they do is to make the flash burst seem longer by rapidly firing the flash bulb many times. This trick can ensure that there is sufficient light to expose the frame at the highest shutter speeds. Shutters which operate at, say, 1/4000 may seem fast, but compared to the speed at which a single electronic flash burst operates, it's an eternity.