I have had this problem with my 3/60 running SunOS as well as my NetBSD/SPARC
IPC talking to my Apple LaserWriter NT, a very slow printer nowadays.
It seems to be (on my serial connection, at least) that the ^D to end
the job gets dropped or otherwise ignored.  Make sure that everything is
set up to pay attention to the handshaking on the //port.  Doing that with
XON-XOF solved my problems up to 30 or so pages.  I suspect that rts-cts
would be best for serial, but I haven't the proper cable.  I really don't
know what to recommend for parallel.
Is there XON-XOF handling in the parallel driver?  Or does it require one
to use HW handshaking?