Epson Stylus Pro 9600 horizontal banding problem

Need help with a Epson Stylus Pro 9600 printing problem.

I work for a printing company and the proofs have a horizontal banding problem. Any proofs which have yellow in the colour is banding. We clean the print head and do a nozzle check. The nozzle check it fine, all nozzles firing correctly. We print the proof and the banding appears. Do another nozzle check and the some of the nozzles are either not working or very distorted.

The history of the machine is this, bought in August 2003, maintenance kit done in 2005, above banding problem appeared in August 2006 and cap assembly and printhead replaced. Above banding problem appeared again in July 2007 and cap assembly and printhead replaced. June of 2008, above banding problem appeared yet again and so far cap assembly replaced, unit where the ink cartridges are held, replaced, banding problem still happening. The next step is the replacing of the printhead. If this happens, it will mean that the printer has had the 4 cap assemblies replaced, four, yes four printheads replaced, one ink cartridge unit changed. All of this before its fifth birthday.

It seems that every 11 months this problem appears and it usually means that the printhead will be changed, even though the nozzles are firing correctly when a nozzle check is done. It is always the yellow which has the problem.

The technician who is servicing the machine spoke to an Epson technician who said it was quite acceptable to replace the printhead every 11 months. Every technician I speak to doesn't seem to agree.

By the time we finally get the machine up and running, the cost of the repair, we could have bought a new machine.

Has anyone else had a problem with one colour not firing correctly when printing, but the nozzle check indicates they are firing correctly, but ultimately, it leads to the printhead being replaced so often. With so many experts on hand, someone I hope has an answer.

Any help would be great.

Thanks
Greg