I have a Canon 30D

I have a Canon 30D and a Speedlite 580 EX. I can not get a consistent and correct exposure with this unit. If I look at the shooting info for example it will show 4 pictures all having the same shutter speed aperture settings yet all pictures will be exposed differently. Some will be overexposed some will be underexposed some may be ok.
I have checked all the settings many times over and talking with other users I think its a inherent problem with Speedlites even the new EX11 hasn't resolved this issue.
Is there anything you can suggest to try to improve the consistency using this unit.

Thank you