I have a couple Sigma lenses including the 50mm f/1.4 DG HSM but I've never had the issue you describe. It's possible it's a firmware issue and that updating the firmware would resolve the issue. You might try sending Sigma customer service an email, or calling them, to find out if this is a known issue.I would rather it be the lens than the camera body.
I haven't seen that lens listed in other member's signature lines Sigma 50mm f1.4 EX DG HSM, so I assume it is fairly uncommon. It would be nice to know if others see the issue.