jQuery Grid Plugin - jqGrid

Using 4.0 with jQuery 1.5.1

When  the row heights are all the same, the AJAX/Virtual scrolling works perfectly.

However, as soon as a row loads that has a different height than the default, the scrolling becomes almost imposible.

Trying to move the scrollbar down to the bottom of the list results in the scrollbar jumping all over the place and loading a bunch of random callbacks.

Not sure what the problem is, but it's very annoying.

Here is a copy of my setup:

            var material = $("#item_list").jqGrid({
                url: "service.ashx?operation=get_list",
                datatype: "json",
                autowidth: true,
                postData: {
                },
                colNames: ['id', 'Code#', 'Date Received',  'PO#', 'Qty Rec', 'Desc', 'Type/Grade', 'Heat#/Lot#/Slab#',
                    'Supplier', 'Special Testing & Notes', 'Mark Detail', 'AttachmentID', 'Attachment Location', 'row_num'],
                colModel: [
                    { name: 'id', index: 'id', width: 25, hidden: true, editable: false },
                    { name: 'CodeNumber', index: 'CodeNumber', width: 50, sortable: true, align: 'center' },
                    { name: 'DateReceived', index: 'DateReceived', width: 90, formatter: 'date', sortable: true, align: 'center' },
                    { name: 'PO_Number', index: 'PO_Number', width: 100, sortable: false, hidden: false, align: 'center'  },
                    { name: 'QuantityReceived', index: 'QuantityReceived', width: 75, sortable: false, hidden: false, align: 'center'  },
                    { name: 'MaterialDesc', index: 'MaterialDesc', width: 250, sortable: false, hidden: false, align: 'center'  },
                    { name: 'MaterialType', index: 'MaterialType', width: 100, sortable: false, hidden: false, align: 'center'  },
                    { name: 'MaterialLot', index: 'MaterialLot', width: 100, sortable: false, hidden: false, align: 'center' },
                    { name: 'Supplier', index: 'Supplier', width: 200, sortable: false, hidden: false, align: 'center' },
                    { name: 'MaterialNotes', index: 'MaterialNotes', width: 200, sortable: false, hidden: true, align: 'center'  },
                    { name: 'MarkDetail', index: 'MarkDetail', width: 200, sortable: false, hidden: false, align: 'center' },
                    { name: 'AttachmentID', index: 'AttachmentID', width: 150, sortable: false, hidden: true, align: 'center'  },
                    { name: 'AttachmentLocation', index: 'AttachmentLocation', width: 150, sortable: false, hidden: true, align: 'center' },
                    { name: 'row_num', index: 'row_num', width: 25, hidden: true, editable: false }
                ],
                scroll: 1,
                rowNum: 10,
                altRows: true,
                mtype: "POST",
                pager: '#item_pager',
                sortname: 'DateReceived',
                sortorder: "desc",
                shrinkToFit: false,
                viewrecords: true,
                rownumbers: false,
                gridview: true,
                height:250,
                caption: "Material Log Items",
                processData: "Loading...",
                editurl: "service.ashx?operation=edit_list",
                onSelectRow: function (rowid, status) {
                }
            });