I have never seen something similar. make a print and use both heads. If problem is on both (black,cyan or magenta, yellow) it´s possibly a problem with sponge (sponge touch heads?) or something in the heads "way". I don´t think its a cleaningproblem it often more gives more randon disturbs.
If you get problem on only magenta/yellow heads a should look at dampers. Another thing i just think about is if you have un-balance in volym between magenta and yellow. If you fill up magenta and yellow is nearly empty you can get stranges result.