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I too had this problem...black prints would not feed properly. the same for fast print qualities.
also, color prints at normal quality wouldnt align perfectly (there would be gaps - white lines - in the print.) the only mode that would work with no problem was color prints at best quality. too slow. recently, the problem became worse and all black printing would immediately cause a paper jam / stall. i had to fix this prob, and found this page.

i just cleaned the encoder disc as suggested by zozo and everything works perfect now.
the disc is a clear wheel with some black alignment figures on it. there is a sensor mounted to read these figures. i wiped both sides of the disc with a slightly moist swab, just to see how dirty it was, and woah, problem solved.

thank you.

You might try cleaning (or replacing if it's really bad) the encoder disc all the way on the left side of the carriage bar. The encoder strip on the back controls horizontal movement while the disc (wheel) controls vertical movement. An alcohol swab is your best bet, though you must have small arms and hands in order to clean it without taking the machine apart.

I have another machine that is doing it as well.