Digital BW, The Print

Hello,
I have had trouble with some part of the printer making track marks on the black areas of prints. They are not pizza-wheel marks (which are a line of small holes) - the marks I am experiencing are a paler track in the black areas, about 1.5-2mm wide.

I have installed the MIS wheel holders, but it's made no difference - so it can't be the front wheels.

Can anyone please tell me what the next thing to investigate is?

This is a major problem because I am currently doing a lot of night photography.

thanks,
Richard
--
http://smallfield.vze.com
http://photos.smallfield.vze.com (Photos web site)
http://warkworth.vze.com/ (Warkworth photo essay)
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   "The chess board is the world, the pieces are 
   the phenomena of the universe, the rules of the 
   game are what we call the laws of Nature.  The 
   player on the other side is hidden from us.  We 
   know that his play is always fair, just and patient.  
   But we also know, to our cost, that he never 
   overlooks a mistake, or makes the smallest allowance 
   for ignorance."
   --Thomas Henry Huxley