B/W prints with Pixma Pro9000

I am new to this forum so be nice to me please. I have not done a forum search. Mea culpa.

I have had my Canon Pixma Pro9000 for a couple of years doing mostly colour work and having no problems. Now I am starting to do black and whites for club competitions and I cannot get unblemished A4 prints with greyscale printing. I get a series of banded lines about 2cm deep across the width within 20% end of the paper. It is not the camera. My images look perfect on screen and print OK in colour, but with a tint. I cannot get a decent black and white using a colour print process.The only way I can get an acceptable A4 sized b/w print is to print in the centre of an A3 sheet - and I think this will mean I cannot do A3 b/w prints. The manual does instruct one to use wide margins with greyscale but does this negate decent sized prints?
I'd be grateful for any sympathy or advice. Regards Peter N.