i see it on mine and i have a 360 connected via vga, also see it on sky via scart( rgb),battlestar galactica shows it up well with its sweeping cameras. i believe its to do with "decay time" of the plasma cell....as in its not switched on and off like an lcd pixel but charged up then the charge is removed, this means that on very fast moving pictures especially when the edge is light next to dark, when the charge is removed the infomation dosnt dissapear it fades.
its more noticable when your watching a black and white film.
its a bit like rainbows on a dlp projector, some people see them some dont and some people do but can live with it.
its not just common to the px60 or even panasonic i`ve seen it on 2 pio 436`s and a hitachi as well ( flame suit on

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