I have had this problem several times. Only with the black, If I print solid black, it does not print a solid black, it prints a much lighter colour, almost like the nozzles are blocked but they are not, ive swapped the head recently with another colour and its still the same, putting a new head has fixed the problem in the past, but it always reverts back to the same problem. If I can get a picture, once I get my other problem sorted i will post it, but has anyone else had a similar type of problem?