Black/grey stripes on sheets with a Ricoh laser...

Hi all,
Some friends of mine own a color laser printer (Ricoh CL7000)...
And have a very annoying problem :
There are some gray/black stripes on the sheets of paper when printing
(in black or black/color). The documents are printed correctly except
from these stripes. They look as "bar codes" and on a A4 sheet, there
are two stripes printed vertically.
I try to clean the printer but w/o any result.
Any idea
TIA
--
JJBee
Pr??sident ?? vie du G.R.O.I.N.
Secr??taire g??n??ral : AluBook