Digital BW, The Print

Some days prints of the same image with the same adjustments and printing
curves come out much denser than others.  I have been experiencing this for
some time and just put up with it by using modified printing curves when it
occurs.

Has anyone else experienced this problem.  Maybe I am doing something wrong.
I swear that one day it changed between prints.  I check that all jets are
working daily and sometimes during the printing session.  Can the driver
somehow have a switch in it that changes the density?  I never change the
driver parameters from the default I have set up.

Bob