How to find out the line that causes the problem?

I have a situation where I cannot duplicate the problem with my ASP.NET application on my development PC. And even on the customer system I get this error 80% of the time but not always.  The error looks as following:

[Null ReferenceException: Object reference not set to and instance on an object]
MyApplication.MyPage.Page_Load( Object sender, Event Args e) + 139
System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e)+15
System.Web.Util.CalliEventHandlerDelageProxy.Callback(Object sender, Eventargs e)+ 34
System.Web.UI.Control.OnLoad(EventArgs e)+ 99
System.Web.UI.Control.LoadRecursive()+47
System.Web.UI.Page.ProcessRequestMain(Boolen includeStagesBeforeAsyncPoint, Boolen includeStagesAfterAsycnPoint) +1062

First I thought that the line MyApplication.MyPage... +139 points to the line in the code causing the problem.  But the PAGE_LOAD does not even have 139 lines.

How can I de ...