From time to time we have System.OutOfMemoryException error on our asp.net web application

From time to time, our web application will cause this error:

System.OutOfMemoryException: Exception of type 'System.OutOfMemoryException' was thrown.

The application is C# ASP.NET 4.5 Web Forms application, running on Windows 2008 R2 server with 6G memory. We have increased memory several times, but it still happens. So I am wondering if its application's problem? What could be the reason?

edit

I have this stack, don't know if it helps at all:

System.OutOfMemoryException: Exception of type 'System.OutOfMemoryException' was thrown.

Generated: Wed, 29 Jul 2015 08:49:24 GMT

System.OutOfMemoryException: Exception of type 'System.OutOfMemoryException' was thrown.
   at ASP.popups_downloadtrendpdf_aspx.__BuildControlTree(popups_downloadtrendpdf_aspx __ctrl)
   at ASP.popups_downloadtrendpdf_aspx.FrameworkInitialize()
   at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at ASP.popups_downloadtrendpdf_aspx.ProcessRequest(HttpContext context)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)