I haven't extensively tested it, but it seems like it may only be doing this on Photo Black prints. Head cleaning then produces a pristine nozzle check, but it may go right back to laying down blobs in the next print. Nozzle check test patterns typically show gaps in the rightmost black grid, and a corresponding pattern of black in the leftmost yellow grid.
The ink is so heavy that I have one image from three days ago where the ink in the blob isn't dry yet. It looks as if someone took a fountain pen overloaded with black ink and drew. Sometimes they are in the midst of the image, more often than not outside of it – above and to the left and right. About every other print (or sometimes every print), I get horizontal and vertical blobs/lines of black ink on the print.
I'm experiencing very frustrating print problems with my Epson r3000 lately.
