jQuery Grid Plugin - jqGrid

I'm getting a recurring error when using celledit.

Firebug stack trace:

jQuery(selector).jqm is not a function grid.common.js (line 58)
viewModal("#info_dialog", Object toTop=true overlay=10 modal=true drag=true)grid.common.js (line 58)
info_dialog("Error", "0 : <br/>error", "Close", "/js/themes/basic/images/")grid.common.js (line 94)
? in grid.celledit.js@189()grid.celledit.js (line 193)
? in jquery-1.2.6.min.js@29(Object, XMLHttpRequest readyState=4 status=0 multipart=false, "error", undefined)jquery-1....6.min.js (line 29)
? in jquery-1.2.6.min.js@28()

I have a page with multiple grids.  Typical setup:
$.extend($.jgrid.defaults, {
        imgpath: gridimagepath,
        datatype: "xml",
        viewrecords: true,
        cellEdit: true,
        cellsubmit: 'remote',
        beforeSubmitCell: appendArgs
    });

$("#gOS").jqGrid({
        url: getURL('load', 'gOS'),
        cellurl: getURL('edit', 'gOS'),
        colNames: colNames2,
        colModel: colModel2,
        caption: "Outside Services - Year " + year,
        afterEditCell: function (row, col, val, iRow, iCol) {
            if (col == 'Contract') $("#gOS").setGridParam({ cellurl:getURL('edit2', 'gOS') });
        }
    });

var getURL = function (op, tab) { return 'HttpRequestPDS.aspx?op='+op+'&pid='+pid+'&yr='+year+'&tab='+tab; };
    var appendArgs = function (rowid, name, val, iRow, iCol) { return {r:rowid, n:name, v:val}; };

The colNames & colModel are pretty standard  - typicall col: {name:'Jan', sortable:false, width:45, align:'right', editable:true, editrules:{number:true} }

The edit actually works, but I get the above error about 30% of the time.

I also get an error popup every time for about a half second - I haven't been able to trap it in firebug. The body of the dialog is "0: error".

Both errors occur after the enter key is pressed. Despite the errors, the value is saved.