Digital BW, The Print

I replaced my yellow ink position on my UTs last week, filling up my 
nearly empty CIS bottles with the replacement (I run two 1290s, both 
now with UTs).
Three days later and I have total clogs in the yellow positions on 
BOTH printers.
I have tried everything I know, even direct injection of windex into 
the print head on one machine.  It is blocked! The windex went 
through all the other head nozzles, but not the yellow.
I get perhaps one or two, sometimes half a dozen nozzles firing on 
nozzle checks, every other colour is clear.
I have left an Epson cart in one machine for two days and get also 
only get one or two nozzles, thats all.  I can thing of nothing else 
to try. Both heads are frozen solid and I am worried that I have two 
dead machines.
What on earth could have caused this?  I did no new CIS re-instal, I 
have tried pulling the ink through the cart from the bottom and it 
flows fine.  The other positions are fine and I have been using UTs 
since they came out without bother.
Have I got a duff batch?
Any one else seen this?  Any ideas at all?

Kind regards

Steve