How to update field value in current item via event receiver

EDIT: I've realized that my approach in the second code block was unnecessary. I could accomplish the same thing by doing the following in ItemUpdated:

SPListItem thisItem = properties.ListItem;

thisItem.File.CheckOut();

thisItem["Facility Number"] = "12345";
thisItem.Update();

thisItem.File.CheckIn("force check in");

Unfortunately, I'm still getting the same error message when "thisItem.Update();" is executed: he sandboxed code execution request was refused because the Sandboxed Code Host Service was too busy to handle the request

I actually was receiving the error above when deploying my sandbox solution originally and used this link (http://blogs.msdn.com/b/sharepointdev/archive/2011/02/08/error-the-sandboxed-code-execution-request-was-refused-because-the-sandboxed-code-host-service-was-too-busy-to-handle-the-request.aspx) to fix it.

I am trying to write a C# event receiver that changes the value of a field when a document is added/changed in a library. I have tried using the following code:

public override void ItemUpdating(SPItemEventProperties properties)
{
    base.ItemUpdating(properties);
    string fieldInternalName = properties.List.Fields["Facility Number"].InternalName;
    properties.AfterProperties[fieldInternalName] = "12345";
}

Unfortunately, this is only working for certain fields. For example, if I replaced "Facility Number" with "Source", the code will execute properly. This may be the fact that we are using a third party software (called KnowledgeLake) that replaces the default edit form in SharePoint with a Silverlight form. Anyway, because I was having challenges with the code above (again, because I think the Silverlight form may be overriding the field after the ItemUpdating event fires), I have tried the following code:

 public override void ItemUpdated(SPItemEventProperties properties)
 {

       base.ItemUpdated(properties);

       //get the current item
       SPListItem thisItem = properties.ListItem;

       string fieldName = "Facility Number";
       string fieldInternalName = properties.List.Fields[fieldName].InternalName;
       string fieldValue = (string)thisItem["Facility Number"];

       if (!String.IsNullOrEmpty(fieldValue))
       {
           //properties.AfterProperties[fieldInternalName] = "123456789";

           SPWeb oWebsite = properties.Web as SPWeb;
           SPListItemCollection oList = oWebsite.Lists[properties.ListTitle].Items;

           SPListItem newItem = oList.GetItemById(thisItem.ID);

           newItem.File.CheckOut();

           thisItem[fieldInternalName] = "12345";
           thisItem.Update();

           newItem.File.CheckIn("force");
       }
   }

First off, the above seems a little klunky to me as I would love to just use the AfterProperties method. Additionally, I am getting the following error when "newItem.Update()" is executed: he sandboxed code execution request was refused because the Sandboxed Code Host Service was too busy to handle the request

Am I missing something here? I would love to utilize the first code block. Any help would be appreciated.