How do I properly use 'yield return' with TaskScheduler.FromCurrentSynchronizationContext()?

I have a long running process in a WCF service that may return a large XML. To avoid running out of memory, I decided it would be a good idea to query the xml in chunks. so I did this:

Task<IEnumerable<XElement>> aTask = Task.Factory.StartNew(() => this.QueryServer());

And the implementation of QueryServer()

private IEnumerable<XElement> QueryServer()
{
    using (var s = new MyService())
    {
        for (DateTime halfADay = StartDate; halfADay < EndDate.Date.AddHours(12); halfADay = halfADay.AddHours(12))
        {
            yield return s.MyServiceClient.DownloadLargeXElement(
                halfADay,
                halfADay.AddHours(12));
        }
    }
}

My problem, is that the UI will hang if I use TaskScheduler.FromCurrentSynchronizationContext() but If I don't use it then my UI won't be updated once it's done querying the XML

aTask.ContinueWith(
    (a) =>
    {
        if(a.IsCompleted)
        {
            foreach(XElement row in a.Result.Select(x => x.Elements()).First())
            {
                DataRow dataRow = this.XElementToDataRow(row);
                MyTable.Rows.Add(row);
            }
        }
    },
    **TaskScheduler.FromCurrentSynchronizationContext()**);

I know this is happening because when it reaches my foreach it then queries the server but at this moment we're back on the UI thread. Is there a better way to go about doing this?