Cisco 501 Pix 6.3.5

I have a 501 running 6.3.5 and when i use the Console and blast by the flash boot and get to a comand prompt i get an error ” cannot select private key ” then the Pix locks up and i have to hard boot the firewall. i have tryed to install a new IOS image ( 6.3.5 ) and it gose great until the reboot and the same issue happens, locks up trying to get anywhere on it.
TIA for any help

Dan