This problem started a few months ago as just the occasional magenta splotch every 10 or 30 pages. However now it is every 2 or 4 pages. Sometimes the magenta splotch is by itself and sometimes it's next to a horizontal line as you see here. That splotch looks roughly the same but can shrink or expand in size. It is always in the same position on the page left to right.
What is interesting is one page which printed the magenta splotch on a line of blue text. That line, which was the first line of the paragraph printed in black whereas the following lines were in blue.
The printer is now printing the horizontal lines as shown. However sometimes those lines are stronger and weaker in colour or in height.
On one page starting from the top going down I have strong black, yellow, cyan and magenta horizontal lines about 1/8" high. These are all 2" apart. On another page, quite possibly the following page, I have the same four lines but they are weak in colour and about 1/16" high.
I have seen the magenta splotch and a horizontal on the electrostatic transfer belt in the cover of the printer. I've taken a tissue and cleaned them off. But they were still present the next time and came back. Note that I was printing in simplex during all this testing and not duplex.
These cartridges are the original cartridges and have never been refilled. The black is at 10% and the colour cartridges are at 34%. I have printed 2546 pages, 1837 colour pages and 595 duplexer pages.