Canon 50mm 1.2L HELP!!!

[shesees]

I was SUPER excited to upgrade in the 50mm department.  I got the Canon 50mm 1.2 L series just last week.  I ordered it brand new from Adorama.  I put it on my 5D Mark iii and tested it around the yard (with my chickens).  LOVED the quality!  I brought it out in the field with me to shoot an intimate wedding ceremony (20 guests).  It worked fine for the first 1200 or so shots.  Now every 5-6th image looks highly overexposed.  But the settings are exactly the same as the other images!  NO flash being used.  I haven't EVER experienced this with any of my other lenses (I have a lot).  Any advice?

ISO 1600  F2.8  1/320sec

[Mitsubishiman]

For what it's worth, I have the same lens, utilized primarily on a Canon 5DsR.

The one variable I do not see mentioned, in the background the sky appears overcast, I have experienced the same effect when set to manual mode, even shots seemingly taken close together. 

With a wide aperture, especially f1.2 I noticed it more, right out of the box.

And especially with an overcast sky, did not seem to see it on a sunny day, so my theory, and correct me if I am wrong, with an ISO of 1600 if the light through an overcast sky suddenly increases then the exposure will be affected, so I switched to auto ISO, and never saw it after.

The 50mm 1.2 L, is not what I have found to be the easiest lens I have to use, it's not like the 24-70 which percentage wise just seems to nail 99% acceptable.

Please relay your outcome as if it is defective, so far mine has been outstanding.