I doubt it.
Sounds much more like an ASP.Net Session issue.
There are 3 models for this:
1. In process - fastest but riskiest as you lose all session data when the AppDomain recycles - which happens more often than you might think.
2. Session State Server - Out of process - in memory of some server (even itself) - slower as you have to serialize the session blob in and out - but safer as you do not lose anything when the AppDomain recycles.
3. Database - Out of process - about the same speed as #2. Even safer as you do not lose anything even if the web server goes down.
Your problem may exist due to #1.
The "Service Unavailable" message is a catch-all when an exception happens during processing of your ASP.NET request. Make sure you set the "mode" attribute on the <customErrors> element in your web.config file to "Off" - the default is "RemoteOnly". This will allow you to see the details of any error that occurs. If you still get the generic "Service Unavailable" message, then it is something outside of your application such as what Joe described above.
Remember to set this attribute back to "RemoteOnly" when releasing your application into production as you don't want users to see the details.
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