Hi,
sorry i jump in this thread with my first post here, but i folow this part of the forum for a while and this thread in particular and i might have a few words to say that helps.
first of all i'm not a photographer, but i print for living. one of my printres is a 9700, not so different as your 9900 photoprinter. for some reasons i use dye inks in this printer, not pigment inks. dye inks are much more thiner than pigments so the clogs are not being produced by pigments accumulating in dampers or head. most of my clogs do occur especialy in the midle of the prints. not if the printer is idle, but in the midle of a very large print with a lot of ink used. after 4 prints of 180g coated paper 1,5 meters by 90 cm i ususly get some clogs.
but i also get clogs on my mutoh draftstation printer both with dyes and with pigments. but not as much as epson 9700.
that is not all, i also have a roland solvent printer. guess what, that clogs to.
all the above printers have various epson heads. for me it is half clear: epson heads do clog by design. more nozzels, more clogs.
but roland user manual describes very well how to take care of your printer. ok, it is solvent , i have to take care of it one, twice a week, but i was very confused that in mutoh and epson manuals thay do not specify any type of deep cleaning. more, for roland i was sugested to replace the whiper ebery 6 months (but again there are different inks). anyway, when i clean the roland i also clean the waterbased printers the way roland told me to clean: clean the whiper, clean carefuly arownd the head, clean the capping station. and cleaning the caping station seems to help the most. there may be debris, hair whatever that prevent the proper sealing of the head and conducts to dry ink and improper cleanings.
i think this was a good approach because i have no missing nozzels and i realy print alot. i have over 5000 pages printed with epson, printed with a RIP, some pages may be over 10 meters long.
i don't know i my sory tells clearly what i wanted to tell: all epson heads clogs. more or less. no matter what ink they carry. i don't think it is heat related, since roland prints even 10 hours without a problem but epson may clog even after 6 sqmeters. a proper maintenance must be done. why it is not specified in the manual it is a mistery to me.
again, sorry for the long story and for my bad english.
Adrian