bi-directional print error (not bi-directional communications error)

[Chuckjr]

Canon S820. Today I started to get missing text lines, or parts of lines. A nozzle check showed all OK except for the "F" box which showed the vertical white lines of death *and* the alignment numbers to the left of the F test box were all missing their bottom halves. I can still print using 'fine' settings and get good results, but draft and normal printing leave these missing lines. I tried the 6-color test jpg on this site and the results were similar: black color bar did not print, and the header text was almost invisible, using normal print setting, but everything printed using 'high quality' settings.
Canon techs emailed me to do deep-cleaning, which did not help. Can anyone tell me what's gone wrong?

Thanks - Chuck