System Folder vanishes

Aaron Workman's two Apple Performas (630CD and 631CD), both running System 7.5.5, showed a strange symptom the other week. When waiting for an incoming fax, the first Mac froze. After successfully rebooting, there were no items listed in the Apple menu and the System Folder had vanished! Numerous attempts to locate it turned up empty. Finally, Norton Utilities FastFind found it but said it was invisible. Using the utility to make it visible again, the System Folder returned and everything was back to normal. Oddly, this exact situation repeated itself two days later on the second Mac.

I have never heard of this happening before. Any one with insight into this, please let me know.