Problem printing black with ricoh gx7700e

hi, i wondered if anyone has had this problem before.... when printing solid black my printer seems to get to a certain point and then pauses and then when resumes printing the black comes out a different tone. Ive tested various files and there is no common pattern, as you can see from the photos some images print solid black fine and others repeatedly have this problem....i have tried to find ricoh specialists that can advise on dye sub ink issues but not having much luck! any advice would be appreciated!http://www.dyesubforum.co.uk/vbforum...tid=3845&stc=1