This issue is seen in a PIX running 7.0.1 software and happens only if there is a memory corruption in the system.
The issue is documented in Cisco bug ID CSCeh90309.
To resolve the issue, upgrade the PIX software version to 7.0.4 and later.
Refer to Software Download: Cisco PIX Security Appliance Software. Click pix704.bin to download PIX software version 7.0.4.