I have a strange problem with yellow contamination, have a look at the nozzle check and the what should be solid Y; its driving me nuts as I've done quite a few strong cleans and nothing any advice would be welcome.
When was the last time you replaced your maintenance station. If it's been more than a year, that's the place to start. As Heyskull said, the pump could be going and/or the cap top could be swollen and causing ink to wick across channels.
Even if you soak your head, without a competent maintenance station, you won't be able to clean the head effectively and even if you could, you're likely to cross contaminate the head again.
One more thought, have you manually checked the ink levels in your yellow cartridge? If it's low and the black is nearly full, this pressure difference can cause the ink to migrate over - but the cap top would be playing a part here as well. Having balanced cartridge levels may alleviate the problem long enough for you to get a job or two out though.
Good Luck