jQuery Grid Plugin - jqGrid

I'm using version 3.6.4. I have several grids in my application and all but one are working correctly. They scroll correctly and pass the correct data back to the server.

I have all the grids set up to use the auto scrolling feature which calls back to the server for more data when you scroll down the grid. When the grid initially loads the data, the server side routine that receives: page, rows, _search, sidx, sord, nd, filters as the query string and receives page=1 which is correct. Then when I scroll passed the frist page (20 records) the value for page is set to 2, which is also correct. If I scroll further the value for page is 2 again, instead of 3. And once its starts to do that the record numbers go negative (i.e. View -9 - 20 of 649). I've tried various different setiings of rowNum but nothing seems to work.

All the grids in my application inherit defaults like so:

            $.jgrid.defaults = $.extend($.jgrid.defaults, {
                datatype: 'json',
                mtype: 'POST',
                height: 'auto',
                imgpath: '/Scripts/themes/ui-darkness/images',
                jsonReader: {
                    root: "Rows",
                    page: "Page",
                    total: "Total",
                    records: "Records",
                    repeatitems: false,
                    userdata: "UserData",
                    id: "Id"
                },
                autowidth: true,
                height: 'auto',
                loadui: "block",
                rowNum: 20,
                //rowList: [10, 20, 30],
                rownumbers: true,
                scroll: 1,
                multiselect: false,
                height: 400,
                viewrecords: true,
                loadError: function(xhr, status, error) {
                    if (error !== "")
                        alert("Error loading data, invalid response from server. " + "(" + status + ")");
                },

                toolbar: [true, "top"],
                sortable: true,

                onSelectRow: function(rowid, status) {
                    RowId = rowid;
                },
                onSelectAll: function(aRowids, status) {
                    GALLERY_WEB.selectAll = status;
                },
                gridComplete: function() {
                    if (GALLERY_WEB.selectAll) {
                        rows = $('.entity_grid').jqGrid('getRowData');
                        $('.entity_grid').jqGrid('resetSelection');
                        for (var x = 0; x < rows.length; x++) {
                            $('.entity_grid').jqGrid('setSelection', rows[x].rn);
                        }
                        var gridID = $('.entity_grid').attr('id');
                        $('#cb_' + gridID).attr('checked', true);
                    }
                }
            });

And the grid that displays the incorrect behaviour is defined like so:

GALLERY_WEB.grid_def_Stock = {
    url: 'GetListData/',
    primaryKey: 'StockNumberID',
    pager: "#pager",
    toppager: true,
    sortname: 'StockNumberID',
    sortorder: "desc",
    viewrecords: true,
    caption: "Stock",
    footerrow: true,
    userDataOnFooter: true,
    colNames: [
                                 ...... column names .......
                ],
    colModel: [
                                 ...... column definitions .......

                ]

};

Have you any idea whats going wrong?

Thanks,

Simon