I have experienced a very similar issue recently with a s370 appliance. Troubleshooting led to identifying an incorrect parameter in the authentication settings on the box. In our case the surrogate timeout had been set to 360 seconds. This seemed to lead to an increased load on our AD servers and after resetting it to 3600 seconds we have not seen one of these five minute outages since. We were experiencing them on a daily basis for a few days before we tried this. Cisco support did not confirm that this was an issue, though it seems like more than a coincidence that the outages started after changing that parameter.
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