查询问题

前台做查询，gridview显示，有分页
查询后有多条记录，分页显示，点击第二页或者后面几页。然后再查询，则程序不能动了，询问原因？
代码如下：        <asp:AccessDataSource ID="AccessDataSource1" runat="server" DataFile="~/App_Data/**.mdb"
            SelectCommand="SELECT * FROM [sfxm1] WHERE ([fymc] LIKE '%' + ? + '%')">
            <SelectParameters>
                <asp:QueryStringParameter Name="YPMC" QueryStringField="textfield" Type="String" />
            </SelectParameters>
        </asp:AccessDataSource>
<input name="ks" type="hidden" />
                          <input name="textfield" type="text" class="unnamed1" value="请输入查询医疗费用名称" />
                          <input name="Submit" type="submit"  class="unnamed1" value="搜索" />&nbsp;温馨提示：采用模糊查询，请输入中文关键字。
        <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" DataSourceID="AccessDataSource1" AllowPaging="true" EnableSortingAndPagingCallbacks="true" PagerSettings-Visible="true" PagerSettings-PageButtonCount="5" PageSize="5" >
            <Columns>
               
                <asp:BoundField DataField="xmbm" HeaderText="编码" SortExpression="xmbm" />
                <asp:BoundField DataField="sfmc" HeaderText="类型" SortExpression="sfmc" />
                <asp:BoundField DataField="fymc" HeaderText="名称" SortExpression="fymc" />
                <asp:BoundField DataField="fydw" HeaderText="单位" SortExpression="fydw" />
                <asp:BoundField DataField="fydj" HeaderText="单价" SortExpression="fydj" />
                <asp:BoundField DataField="type" HeaderText="等级" SortExpression="type" />
              
            </Columns>
        </asp:GridView>后台接收 public string sfmc;
    
    protected void Page_Load(object sender, EventArgs e)
    {
                if (Request.QueryString["textfield"] != null)//判断是否有接受值，如果有
        {
            sfmc = Request.QueryString["textfield"].ToString();
        }
    }