Canon 580EX mkI problem

I have a 580 that I been using for a long time. I noticed the last few shoots that it underexposes by 1.5 - 2 stops almost every shot.

I just bump up the FEC up on the camera but I shouldn't have to do that. I get the same results with my 5D2 and my 7D.

Am I doing something wrong that is bringing this on? I can't find the owners manual to see if I changed some setting without knowing it or does it need repair.

Thanks for any help...
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