PIX: Failover message block alloc failed

Hardware:   PIX-515E, 128 MB RAM, CPU Pentium II 433 MHz
OS: Cisco PIX Security Appliance Software Version 7.1(2)

I have two of the above firewalls that are connected with a failover LAN connection.  Every couple of months the Primary firewalls beings to deny any new connections through the firewall and I cannot SSH or even console into the firewall.  When I try to console into the FW it scrolls all the logging messages and will not allow me to login.  I get a weird "command...." message after issuing the "enable" command and get sent back to the user exec prompt.  
I have a syslog server running and after that keeps displaying the following message:
"(Primary) Failover message block alloc failed"

Here is Cisco's explanation of the message:
Explanation: Block memory was depleted. This is a transient message and the PIX Firewall should
recover. (Primary) can also be listed as (Secondary) for the secondary unit.

After about 20-30min of no connectivity and the above messages the Primary FW eventually fails over to the secondary FW (on its own) and connections are restored.  I don't see any other issues in the log other than the message above.  
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Pete LongConnect With a Mentor Technical ConsultantCommented:
Yeah I take your point - I know its a bit of a cop out answer (sorry) but you could try an upgrade to version 8 (cause if you opened a TAC case they would suggest that first)

I'm assuming that the firewalls fail over properly when this happens?
Pete LongTechnical ConsultantCommented:
Your PIX is getting Overloaded, Along with the show cpu usage command, you can use the show blocks command in order to determine how often the PIX is overloaded

Pete LongTechnical ConsultantCommented:
nakoz69Author Commented:
Awsome document!  I never even knew what packet-processing blocks were.  However, there is still one more issue, i cannot access the firewall when it is generating the block errors.  Even trying to console in it will not allow me to login and check the blocks or CPU.
nakoz69Author Commented:
Upgrading to version 8 fixed the memory issues. The Pix has been running clean for months now with no issues.  Cisco admits there are memory problems with 7.1(2)
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