API of Cisco Prime block me Access

Need to monitor all devices that are connected to the Prime infrastructure in proactive.  For that got a  script, this will run every 5 minutes to get information. 

•      The first Section connect to the API and get all devices with status Unreachable.
•      The Second section connect to the API and get all new events that contain "is down" into description
•      The Third section connect to the API and get all new events that contain "is up" into description.

      This means got 3 connections every 5 minutes to the API. 

•      But when  tried  to make the script run every 10 minutes  or 15 minutes is showing Error. 
•      When the script starts everything goes well. But after 30 minutes when tried to connect to the API, I receive this error :

      The remote server returned an error: (401) Unauthorized

• Can you confirm: you are receiving successful responses (HTTP status 200) for about 30 minutes and then the 401 errors start happening. 
• What version of Prime Infrastructure are you running.
• Do you have TACACS, SSO, or any other AAA provider enabled on Prime Infrastructure.
• Can you include the body of the response you get from the server when the 401 happens.

just changed the request by HttpWebRequest in Asp.net

     And added  KeepAlive = false

 And now everything work fine.