I've been having this issue with my workstations for about four months
now. It doesn't seem to be triggered by anything I can find. At first I
thought it was the version of the iPrint client I was using. All of my
users right now are on 4.12. I started seeing this problem when we
upgraded to 4.11 but not everywhere. For those having the problem I
back-reved there client to 4.05 and that seemed to fix this issue, for a
while. Then it came right back. Once 4.12 came out I tested it, felt
that it had solved my problems and pushed it out to my users. Everything
was running fine for a while again and certain users started having the
problem of application going "Not Responding" again. One side step (work
around) I have found to this is to install the same printer with the same
driver to IP (Standard TCP/IP Port). Once I do that, printing works great.
It must be something with the iPrint client. Not all users sharing the
same printer have this issue and none of the users having this problem
seem to have unique applications. The other strange thing is that a user
that has printed fine for months could start having the problem tomorrow.

Any help will be greatly appreciated.