PostScript errors are usually generated by one of three things: a corrupt document, a bad printer driver, or a slight incompatibility between the version of PostScript in the printer and what the application thinks is there. Because you can print text without a problem, we suspect the latter of the three, particularly if you're printing to the original LaserWriter. To verify this, you may want to try to print to a newer printer, like a LaserWriter IINT.
If you're interested in getting deeper into Postscript troubleshooting, get some of the reference manuals on the subject. Adobe has books on the subject, as do other publishers. For more information, search on "Adobe" AND "3rd".