Weird behavior for Form Authentication and ASP.NET membership

My web site is keep pushing the logged in users out to login page after around 5 to 7 days from using the system, where the problem resolved temporarily after making IIS-Reset and return back after another 5 to 7 days.
I have:
-          ASP.NET web site built on .net framework 4.0/c#
-          Web.config configuration as below:
<membership defaultProvider="AspNetSqlMembershipProvider">
      <providers>
        <clear />
        <add name="AspNetSqlMembershipProvider" type="System.Web.Security.SqlMembershipProvider, System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" connectionStringName="LocalSqlServer" enablePasswordRetrieval="false" enablePasswordReset="true" requiresQuestionAndAnswer="false" a ...