Just curious if any solution came out of this. I, too, am having the exact same problem with version 10.5, using w2k, hp750c plus, driver hpgl 4.63, with times new roman text. My lineweights are sometimes affected, as well as any fills showing up as black instead of white, or not plotting at all. There really doesn't seem to be any rhyme or reason to the problem. It's very inconsistent.
Any help or suggestions would be much appreciated!