thare are many things that can cause the effect you mention
in digi imaging circles we call it posterisation [as against banding, which is shown as parallel straight lines due to paper feed irregulatities]
posterisation: not enough steps are being used to reproduce the colour in your file as continuous tone, thus you see steps.
this could be caused by a bad printer profile
Good advise from Neil as usual, but it may be worth running your cursor set to 3x3 over the problem area and watching the info read out. If you have clear steps in the numbers it's in the file, otherwise it's a printer/profile problem.
Cheers
Richard -- Richard Kenward www.precision-drum-scanning.co.uk
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