I checked the grid declaration and the error information you posted and it looks more like a general kind of error not directly related to the control.
With your setup the grid should not perform any kind of ajax request or client-side callbacks. What is strange here is that the Page_Load event should fire in all cases, even if there was ajax request.
The error in your case seems to originate from the RaisePostBackEvent handler, which should also fire after the server-side Page_Load event. You can verify this by handling the RaisePostBackEvent of the page:
//check the values of sourceControl and eventArgument should not be null
The most likely reason for the exception in this case is that any of the arguments in the above method is null. It is not clear what might be the cause of the exception but it seems to be related to the ASP.NET framework and Sharepoint rather than some specific code in the grid.
To further troubleshoot the issue you can check any server-side code or additional markup on the page that is likely to cause problems during postback. Another option is to try to isolate the issue in an as simple page as possible (containing just grid with dummy data) and see if the same problem is still replicated there.
The fact that the issue does not happen regularly on each postback again leads to the conclusion that is more of environment related problem rather (for some reason the ASP.NET does not pass a valid argument to the RaisePostbackEvent method) some problem in the grid's code.
You can further troubleshoot to see if the problem occurs under certain circumstances or specific setup that causes the exception.
From the exception details it seems you are running a Sharepoint Foundation edition which has a limited set of features and may cause problems in some more complex scenarios. It might be a good idea to check if the same issue is replicated on the full Sharepoint Server version.
I hope the above suggestions help. Let me know if you have any other questions.