What do you mean exactly by shutter variation?? Or do you mean shutter vibration? What shutter speeds are you shooting at? Too low and you'll naturally get some vibration...even with good hand holding technique. Of course, to eliminate most or all vibrations, a tripod is a must.
Could you fill out the rest of your profile info, such as where you're from, gear used, etc. It helps us help you with any questions you might have. Thanks.
Just a wild guess. Make sure your not on the "Bracket setting". This would give you one OK shot, one dark and then one light… It happened to me before and don't even ask me how it did.
Marcel, your wild guess is spot on, yes the problem proved to be the bracket setting that was obviously adjusted.
Thanks for prompt feedback.
JohnT, Ireland