Question uneven prints with vertical banding on Roland RF 640

Hi there, i have a difficult job that drives me crazy . Have to print big gray squares with some white parts, but after printing about 1m, the printer starts to print the gray surface uneven. I made some tests with prints and found out, that a clean CYAN-Square ist printed uneven too. Ist starts ok, then it would be darker - looks like an color gradient. After heavy cleaning it looks better but when start printing the signs it comes back: uneven grays with irregular horizontal banding. Anybody there with some experience in this error?

P.S. Someone out there with a service manual for the RF 640?