canon w6400 pigment lines on print

[mossie]

hi all i have a w6400 pigment printer and am new to this printing business.yesterday while printing a 2ft / 4 ft poster two coloured vertical lines appeared full length on one side of the finished poster.i changed the paper and same thing, ran both the A and B printhead cleaning,printhead alignment ,and no good.the rest of the poster is perfect except these two lines (yellow and light blue).any help would be greatly appreciated as im considering giving up on this machine.ps canon support is rubbish and i certainly would not look at another canon machine again regardless of price ,quality etc.....