My issue with the Xerox plotter was solved, it was a plotter issue. I am going to attept to recreate the problem with our HP design jet using the new built in gimp driver in Mac OS X 10.3.
Remember that Gimp is a printer driver and so does not serve as a real test of plotting problems.
We have an HP1050c which is also only plotting one set at a time. This hasn't been a big problem for us because we mostly plot one-offs for review or use PlotFlow for multiple sets (this has always been my preference anyway). Big jobs we send out to a service bureau.
This machine also has all sorts of problems printing from OSX (from any program, not just AC & PM). The HP service tech said that it is a problem with the HP OSX driver for this particular machine and that it would not likely ever be fixed. Fortunately we are due to replace the thing soon.