I lost 2 3800's to black ink migrating into the yellow. The first one I bought new when they were first released, the second one a refurb.
The first one lasted around 5 years, the second one just over a year. Both were used moderately. Sometimes printing a couple hundred prints in a day
sometimes going weeks without use.
My guess is somewhere either in the head or ink delivery system a leak had developed allowing black ink into the yellow channel. With my second 3800
I completely ran out of black ink without the printer telling me it was low, because black ink had leaked into the yellow channel, that was not accounted for.
The first one I trashed because I didn't think it was worth the expense to repair a 5 year old printer. The second, I had a time crunch and felt I could be back up
and running faster by buying a new 3880. So far, so good on the 3880.
If you decide to get it repaired, I would be interested in what the exact issue is. That is, if they can figure it out without just throwing parts at it until one sticks.
There is definitely an issue with the 3800, I would say it is time to either have it repaired or get a new printer.