Single Sign-On ISS 6.2 - One of two sites Intermittently asking for credentials

Question

Currently in our testing environment, one IIS site (out of two) intermittently asks for credentials.

Our monitoring responds with 401 errors (believe due to login prompt) - shown below: (shortened)

GET / - 80 - Microsoft+Monitoring+Agent+7.1.10184.0 - 401 2 5 0

Both sites are configured for SSO, the other site works fine all the time, even while the affected site is having issues.

This issue is intermittent - occurred 2 days out of 7 this week.

We currently have a workaround of restarting only the affected site, which then allows users to access via SSO,

and we even have an automated task for this.

However I would still like to resolve the root issues itself. Event gaining a greater understanding of how a user SSO request can be transformed into needing credentials, and what common causes of this may be. Any help is greatly Appreciated.

Additional info:

-IIS version 6.2 used

-Windows server 2012 R2 used