You may start having issues with certain users who are using ISA 2006 as there web proxy on certain web sites which cause IE not to work until it is restarted.
A possible cause for this is ISA’s Flood Mitigation settings which limit you to 600 http requests per minute. Certain websites will make more request than this limit and ISA will block http requests from the IP address and you will have this error appear in the alerts tab of the ISA.
“The number of HTTP requests per minute from the source IP address *.*.*.* exceeded the configured limit. ISA Server will block new HTTP requests sent from this IP address.”
To stop this issue occurring you can add exception to the flood mitigation settings which will increase the maximum number of http requests to go up to 6000.
To add an Exception to Flood Mitigation Settings
1, Expand configuration and press the general option on ISA management console.
2, Press “Configure Flood Mitigation Settings”
3, Go to the IP Exceptions Tab and add in the computers which are experiencing the issues and apply the rule and you have then increased the HTTP Requests per minute value to 6000.
Hope this helps