Since it seems that you are downloading quite a few fonts to the printer, we would suggest enabling "Unlimited Downloadable Fonts" in the Page Setup Options dialog. This option purges each font from the printer's memory after it is used. As a result, print jobs will probably take longer to process, however, this should get rid of the PostScript errors.
If you will be constantly downloading fonts to the printer, you might want to purchase additional RAM, or even a hard drive for the printer to store the TrueType fonts.