strange horizontal line on some bluray movies

[stickymcglue]

hi !
can anyone tell me what this is !?
I am running a panasonic blu ray player into a viera plasma using a HDMI link.
obviously a stunning picture most of the time , I say most of the time because during "i am legend" and " 10000 bc " I noticed a permanent horizontal line in the middle of the screen during dark scenes , its as if the picture has been made up of two seperate transfers !! both these are warner disks so i tried sudden impact ( also a warner disk ) but thats perfect in dark scenes.
Anyone got any ideas what this is ? is it a mastering fault ??
a bit bemused to be honest !
any info appreciated...
Jon