Inconsistant IE8 ajax post success from MVC2

Question

I am getting an incosistant error in IE8, it happens about half the time. I have cascading drop downs using ajax to fetch the next select box options.

 $(function () {
    $("#stock_item_id").change(function () {
        var stock_item = $("#stock_item_id > option:selected").attr("value");

        $.ajax({
            type: "POST",
            contentType: "application/json; charset=utf-8",
            url: "/Unit/ForStockItem/" + stock_item,
            data: "{}",
            dataType: "json",
            success: function (data) {
                var options = '';
                var selected = "0";
                for (p in data) {
                    var unit = data[p];
                    if (unit.selected == "true") { selected = unit.id; }
                    options += "<option value='" + unit.id + "'>" + unit.name + "</option>";
                }
                $("#unit_id").removeAttr('disabled').html(options);
                $("#unit_id").val(selected);
            }
        });
    });
});

And the MVC2 controller action

public JsonResult ForStockItem(int id)
    {
        StockItemRepository stockItemRepository = new StockItemRepository(repository.db);

        tblStockItem stockItem = stockItemRepository.Single(id);
        return new JsonResult
        {
            Data = repository.parentList(stockItem.tblUnit.BaseUnitId, stockItem.unit_id)
        };
    }

It works 100% from firefox, chrome etc and about half the time in IE8. When it doesnt work in IE8 data variable is null in jquery.

If I change the request to a GET and add JsonRequestBehavior = JsonRequestBehavior.AllowGet to the JsonResult it works 100% in IE8 (no other code change). While this is a solution it is not ideal. Is this a known bug with Jquery and IE8, what can I do to fix it and still use POST?

Many Thanks