The issue may be due to following reasons
PIX reboots with stack trace in isakmp_receiver thread (stress).
The Boothelper installation only supports PIX Firewall Version 5.1, 5.2, 5.3, 6.0, and later. After Boothelper downloads the PIX Firewall image via TFTP, it verifies the checksum of the image. If it is not Version 5.1 or later, it displays the message "Checksum verification on flash image failed" and reboots the PIX Firewall