docucolor 240 yellow shadow / colour off register ?

[ajs]

My docucolor 240 has developed a problem a few days ago, where anything i print whether it be full colour or mono theres a yellow shadow as if the yellow is off register, does this whether i print just black or full colour,
but if i print black text on an image with colour the black text is fine

i have been across forums for a few days and tried the following -

swapped colour drums
new black drum
new charge corotron
cleaned ros glass

ran belt edge learn diagnostic
ran registration cycles in diagnostics

all to no avail

has anyone got any other idea ?
would be greatly appreciated