Digital BW, The Print

Has anyone had problems with the "Yellow" or lightest gray in the UT 
set clogging. I've been having quite a problem with this one ink. I 
get good nozzle checks and then, half way through a print I can see 
the light gray fail somewhat. When I do another check the yellow has 
broken lines. I do a cleaning or two and then I might have a good 
check again. I can have a few prints and then again, the yellow clogs.
Is it the head or the ink? I'm using QTR with a 7500. I've seen some 
posts lately about the possible settling of pigment

John H