Your Power Macintosh 9500 is technically not freezing, it is pausing as ARA examines the serial port during the shutdown process. Since the pause may take up to 10 minutes, the computer certainly could appear to be frozen. Upgrading your system software with System 7.5 Update 2.0 will solve this issue.
NOTE: This issue seems to occur when using an external modem and not when using a GeoPort. This is because the GeoPort uses its own drivers, while an external modem communicates through the serial port drivers which is causing this issue.