Digital BW, The Print

Has anyone had a Canon IPF6300/8300 or similar with a banding problem they could not resolve? 

Our IPF6300 started banding about 6 months ago, we changed a head and it worked fine for a couple hundred prints over about 2 months, then it started banding again in the darker tones on both color and B&W prints. The banding is difficult to see and the printer always does a good nozzle print.

Canon Techs suggested changing the newer head again, which they supplied because it was still in parts warranty. It didnt help so they suggested replacing the second head which also did not help. Now after buying two heads and several hundred dollars in ink through numerous cleaning cycles and tests they say we need to schedule a $1000 service call. 

Before doing that however, I see a couple of possible faults. 

Canon told us it is not possible to print only one pure color at a time to determine which head or ink lines may be at fault as I initially thought that bubbles in lines may be causing some trouble. I now think that the circuit board components responsible for squirting ink through a particular set of holes may be defective and therefore looks like a head defect or perhaps the one of the feed mechanisms are binding or not keeping in sync with the ink squirting.

We have tried different settings, drivers etc and nothing helps so if anyone here has had a similar experience I would like to hear about it.

Larry