To reproduce:
- Use the code below on a new grid with 3 combo box columns - computer_id, computer_des, location_id

public static DataTable dsComputerImages = null;
public static DataTable dsLocations = null;

private void loadComputerImagesGrid()
{
    dsLocations = new DataTable();
    DataColumn newColumn = new DataColumn("location_id", typeof(Int64));
    dsLocations.Columns.Add(newColumn);
    newColumn = new DataColumn("location_des", typeof(String));
    dsLocations.Columns.Add(newColumn);

    DataRow newRow = dsLocations.NewRow();
    newRow["location_id"] = 1;
    newRow["location_des"] = "Boston";
    dsLocations.Rows.Add(newRow);

    newRow = dsLocations.NewRow();
    newRow["location_id"] = 2;
    newRow["location_des"] = "New York";
    dsLocations.Rows.Add(newRow);

    newRow = dsLocations.NewRow();
    newRow["location_id"] = 3;
    newRow["location_des"] = "Huston";
    dsLocations.Rows.Add(newRow);

    dsComputerImages = new DataTable();
    newColumn = new DataColumn("computer_id", typeof(Int64));
    dsComputerImages.Columns.Add(newColumn);
    newColumn = new DataColumn("computer_des", typeof(String));
    dsComputerImages.Columns.Add(newColumn);
    newColumn = new DataColumn("location_id", typeof(Int64));
    dsComputerImages.Columns.Add(newColumn);

    newRow = dsComputerImages.NewRow();
    newRow["computer_id"] = 1;
    newRow["computer_des"] = "AAA";
    newRow["location_id"] = "1";
    dsComputerImages.Rows.Add(newRow);

    newRow = dsComputerImages.NewRow();
    newRow["computer_id"] = 2;
    newRow["computer_des"] = "BBB";
    newRow["location_id"] = "1";
    dsComputerImages.Rows.Add(newRow);

    newRow = dsComputerImages.NewRow();
    newRow["computer_id"] = 3;
    newRow["computer_des"] = "CCC";
    newRow["location_id"] = "2";
    dsComputerImages.Rows.Add(newRow);

    newRow = dsComputerImages.NewRow();
    newRow["computer_id"] = 4;
    newRow["computer_des"] = "DDD";
    newRow["location_id"] = "3";
    dsComputerImages.Rows.Add(newRow);

    this.ComputerImagesGrid.AutoGenerateColumns = false;
    this.ComputerImagesGrid.DataSource = dsComputerImages;

    this.ComputerImagesGrid.AutoSizeRows = true;
}

private void ComputerImagesGrid_DataBindingComplete(object sender, GridViewBindingCompleteEventArgs e)
{
    ((GridViewComboBoxColumn)((RadGridView)sender).Columns["location_id"]).DataSource = dsLocations;
    ((GridViewComboBoxColumn)((RadGridView)sender).Columns["location_id"]).ValueMember = "location_id";
    ((GridViewComboBoxColumn)((RadGridView)sender).Columns["location_id"]).DisplayMember = "location_des";
    ((GridViewComboBoxColumn)((RadGridView)sender).Columns["location_id"]).DisplayMemberSort = true;
}