streaming problem and thread freeze

Hello,
I have a client program that read an XML from a STREAM every 60 seconds.
From the main Thread I do an infinite loop where I create a new working thread
executing all the operations: read, parse, etc..

This is what I do:

<code>
 DownloaderTask tempTask = null;

        while (true) {

            if (tempTask != null) {
                if (tempTask.isAlive()) {
                    tempTask.cancel();
                }
            }

            tempTask = new DownloaderTask();

            tempTask.start();

            try {
                sleep(frequency * 1000);
            } catch (InterruptedException ex) {
                log.severe("Main sleep failed");
            }

        }

</code>

Inside the Run method of the DownloaderTask, I first read from the Stream (a class field) with a BufferedReader, in this way:

<code>
 private String readDocumentFromStream(BufferedReader reader) throws IOException {
            char[] buffer = new char[4 * 1024];
            int charsRead = -1;
            String retVal = "";
            log.finer("---START reading XML ---");

            while ((charsRead = reader.read(buffer, 0, 4 * 1024)) != -1)
            {
                log.log(Level.FINER, "CharsRead: {0}", charsRead);
                retVal += String.copyValueOf(buffer, 0, charsRead);
            }
            log.finer("---STOP reading XML---");

            return retVal;
        }
</code>

In the DownloaderTask class I wrote a cancel() method, called by the main thread in the case the working thread was still alive after the sleep().

It's definition:

<code>
   synchronized public void cancel() {

            this.interrupt();
            try {
                if (rd != null) {
                    rd.close();
                }
            } catch (IOException ex) {
                log.severe(ex.getMessage());
            }

            http_conn.disconnect();

            log.log(Level.SEVERE, "Timeout. Thread {0} is being canceled and resource released.", this.getId());

        }
</code>

My problems.
 
Sometimes I see my program active, doing nothing, blocked probably in the read(), because if I check all logs, the last line written is "CharsRead: xxx" and no more.
Here I post one of my saved log (where the read buffer was 1024 bytes):

<LOG>
||-- 2011-05-03 06:44:59 FINE: Updating events data... --||
||-- 2011-05-03 06:44:59 FINE: Events updated! --||
||-- 2011-05-03 06:44:59 INFO: Release date: 2011-05-03 06:45:15 | Received: 187 Inserted: 0 Updated: 0 Deleted: 0 ||| Errors: 2 --||
||-- 2011-05-03 06:44:59 FINER: Normal closing thread 1.723 --||
||-- 2011-05-03 06:44:59 FINER: Thread 1.723 Closing --||
||-- 2011-05-03 06:45:52 FINER: Thread 1.725 Opening --||
||-- 2011-05-03 06:45:52 FINE: Opening server connection to: mysite.com:80 --||
||-- 2011-05-03 06:46:19 FINE: Reading From Stream --||
||-- 2011-05-03 06:46:19 FINER: ---START reading XML --- --||
||-- 2011-05-03 06:46:19 FINER: CharsRead: 1.024 --||
||-- 2011-05-03 06:46:19 FINER: CharsRead: 1.024 --||
||-- 2011-05-03 06:46:19 FINER: CharsRead: 1.024 --||
||-- 2011-05-03 06:46:19 FINER: CharsRead: 929 --||
||-- 2011-05-03 06:46:19 FINER: CharsRead: 1.024 --||
||-- 2011-05-03 06:46:19 FINER: CharsRead: 1.024 --||
||-- 2011-05-03 06:46:19 FINER: CharsRead: 872 --||
</LOG>

I noticed another strange thing in this log, that is I can't see the Thread Id: 1724.
Can you help me to find where I am wrong? In the logic or maybe in the syntax of the program?

Thanks in advance,
Flavio