1. What size load capacitors are you using with your crystal?

2. Have you used an oscilloscope to verify that your oscillator is running at approximately the speed you expect it to be?

3. Do you have any other crystals that you could use that don't have the possibility of generating serial errors (29.4912, 22.1184, 11.0592, etc.)?

4. What revision of the DS89C420 are you using? There should be a six digit code on the top of the chip of the form yywwxx, where yy is the year of manufacture, ww is the work week of manufacture, and xx is the revision code.