If you have intermittent internet connectivity, your Cisco PIX can become extremely annoyed and decide to munge up it's security key(s) for remote access. You need to regenerate your rsa key(s).

If you can't access the device remotely, you must use the serial console. (blue cable) Attach it to a serial port on a workstation and launch the terminal emulator on your system.

Boot up the PIX.

Login to configure terminal mode.

Type:
'ca zeroize rsa' -enter-
'ca generate rsa key -keysize-' -enter-
'ca save all' -enter-

Where -keysize- is a size in bits, eg 1024.

Reboot the device:
Type: 'reboot' -enter-

Enjoy!