I've been discussing this with Jon, he recommended availing myself of the wisdom of the forum (Dana in particular).
I left my 9900 off over a 3 week vacation. Came back, started up, ran a power clean and seemed to get it in working order. I did several 42 inch wide prints without a hitch. Within a month, I was having problems with C and VLM clogging, especially the latter. At first, there were just a few displaced or broken lines in the nozzle check, and nothing detectable when printing a gamut. Printing something after cleaning would usually produce a good nozzle pattern, but the clogs were way too frequent and the LCD console would report clogs even when I couldn't see anything on the check pattern.
The printer is set to autoclean, but I run a nozzle check before every print run, nearly every day. It's on all the time, so it doesn't auto clean all that frequently. As of now, autoclean, clean or manual pair clean (for the VLM) fails. I can't see that running it repeatedly will be effective, and I've read many cautions against doing several power cleans.
Jon suggested changing the wiper blade. I did that about two weeks ago, and it seemed to improve matters. However, the problem has returned, and is worse than ever on VLM. In addition to several broken lines in the nozzle check, I get faint but definite banding in printing a gamut. A loupe reveals dots of ink misplaced, beyond the edge of the print area.
Jon also suggested soaking a Brawny towel with PiezoFlush and then slipping it under the print head and soaking it for 20 minutes or so, to clean the surface of the heads. I've ordered a small bottle of PiezoFlush, and am waiting for it to arrive. I used to do something like this with my old Epson Stylus 3000 (though flush cartridges were always more effective). I've never done it with a 9900, and would appreciate some instructions.
The printer is 4 years old, and has done about 1500 prints, usually in medium to large format. I use it weekly, at times daily. I've been using the ConeColor Pro inkset since last year, although I still have Epson O and G cartridges installed.
If the soaking doesn't work, I wonder what my next step should be. Purchase the PiezoFlush kit? replace the dampers? the heads? the printer?
Advice is welcome. Instructions on soaking the print head (or my own) are also welcome.