ASA 5520 with multiple contexts becomes unresponsive

Question

Hi all. We have encountered a perculiar problem with a pair of our ASA 5520 firewalls with 2 contexts(each context being active on different ASA). What we are seeing is that sometimes when we have a sudden increase of inbound traffic(mostly HTTP) towards servers behind the firewalls they seem to go bananas for the lack of a better expression.
They become unaccessible via ssh and the traffic drops significantly. The problem is mitigated by disabling one of the monitored interfaces for failover(on one of the switches the firewall is connected to) so that both contexts become active on one firewall. After that the firewalls seem to come to their senses and we can enable the switch interface again but sometimes one of the pair needs to be rebooted to restore full funcionality.
To us it seems like there is a problem with failover and contexts but we haven't been able to pin it down. The failover link isn't stateful and when we tested the failover it works fine both ways with each ASA taking up the full load when the other ASA of the pair is not available.
Did anyone come across a similar situation with their firewalls?

Answer 1

We are using ASA version 8.2(5).
The configuration of the failover is:
failover
failover lan unit primary
failover lan interface fail_int GigabitEthernet0/3
failover interface ip fail_int x.x.x.x 255.255.255.252 standby x.x.x.x
failover group 1
  preempt
failover group 2
  secondary
  preempt
Output of the "show failover":
  This host:    Primary
  Group 1       State:          Active
                Active time:    399409 (sec)
  Group 2       State:          Standby Ready
                Active time:    111 (sec)
                slot 0: ASA5520 hw/sw rev (2.0/8.2(5)) status (Up Sys)
                  admin Interface out (x.x.x.x): Normal (Waiting)
                  admin Interface inside (x.x.x.x): Normal (Waiting)
                  admin Interface dmz4 (x.x.x.x): Normal
                  admin Interface dmz1(x.x.x.x): Normal (Not-Monitored)
                  C1 Interface out (x.x.x.x): Normal (Waiting)
                  C1 Interface inside (x.x.x.x): Normal (Waiting)
                  C1 Interface dmz5 (x.x.x.x): Normal
                  C1 Interface dmz1 (x.x.x.x): Normal (Not-Monitored)
                slot 1: empty
  Other host:   Secondary
  Group 1       State:          Standby Ready
                Active time:    0 (sec)
  Group 2       State:          Active
                Active time:    398992 (sec)
                slot 0: ASA5520 hw/sw rev (2.0/8.2(5)) status (Up Sys)
                  admin Interface out (x.x.x.x): Normal (Waiting)
                  admin Interface inside (x.x.x.x): Normal (Waiting)
                  admin Interface dmz4 (x.x.x.x): Normal
                  admin Interface dmz1(x.x.x.x): Normal (Not-Monitored)
                  C1 Interface out (x.x.x.x): Normal (Waiting)
                  C1 Interface inside (x.x.x.x): Normal (Waiting)
                  C1 Interface dmz5 (x.x.x.x): Normal
                  C1 Interface dmz1 (x.x.x.x): Normal (Not-Monitored)
                slot 1: empty
Stateful Failover Logical Update Statistics
        Link : Unconfigured.
When I disabled the monitored interface it was always the same interface altough I believe the same effect could be achieved with disabling any of the monitored interfaces.
As for memory and CPU when it happens I cannot access the units to get a reading but I asume it's through the roof. 
The thing that troubles me more is that the situation persists when the load drops and I have to perform the solution from the first post. One would assume that with the drop of the load that both firewalls would start to behave normally.
And I see that I haven't mentioned it before but when the load drops both units continue to handle traffic normally but I sometimes see as a side effect that I cannot SSH to one of the units. That unit usually has to be restarted.