bizhub c500 registration

[otomatic]

hello all

I've just done a 200k pm and all was well
until running some cover stock
the first copy looks great but soon after the colors are off register
the yellow being the worst, as much as a 1/16 inch off

I'm just guessing the belt is getting slack in it with the thick paper
is there something I missed with this one

it wasn't doing this before

thanks