[SOLVED] AsyncTask HttpPost execute fails on 3G, but works on Wifi

I need to do a Http post of some strings to a web service. I am using KSoap. (partly based on this answer by kuester2000)

@Override
protected JSONObject doInBackground(JSONObject... params) {
    String result;
    
    HttpParams httpParameters = new BasicHttpParams();
    // Set the timeout in milliseconds until a connection is established.
    // The default value is zero, that means the timeout is not used. 
    int timeoutConnection = 30000;
    HttpConnectionParams.setConnectionTimeout(httpParameters, timeoutConnection);
    // Set the default socket timeout (SO_TIMEOUT) 
    // in milliseconds which is the timeout for waiting for data.
    int timeoutSocket = 50000;
    HttpConnectionParams.setSoTimeout(httpParameters, timeoutSocket);
    
    HttpClient httpclient = new DefaultHttpClient(httpParameters);
    setupClient(httpclient);
    //HttpURLConnection httpclient = new HttpURLConnection(_url);
    
    Log.d(TAG, "Posting to ["+_url+"]");
    HttpPost postdata = new HttpPost(_url);
    
    ...set up parameters
    
    ResponseHandler<String> handler = new BasicResponseHandler();  
    Calendar c = Calendar.getInstance();
    long start = c.getTimeInMillis();
    try {               
        Log.d(TAG, "Starting post data call ["+_url+"]");           
        result = httpclient.execute(postdata,handler);
        c = Calendar.getInstance();
        long total = c.getTimeInMillis() - start;
        Log.d(TAG, "Finished post data call ["+result+"] in ["+total+"] millis");
    }catch(HttpResponseException e)
    {
        c = Calendar.getInstance();
        long total = c.getTimeInMillis() - start;
        Log.e(TAG, "HttpResponseException: ["+total+"] millis. There was a problem communicating: " + e.getMessage());
        return null;
    }catch (ClientProtocolException e) {
        Log.e(TAG, "ClientProtocolException There was a problem getting to the service: " + e.getMessage());
        return null;
    } catch(SocketTimeoutException e){
        Log.e(TAG, "SocketTimeoutException There was a problem connecting: " + e.getMessage());
        return null;        
    } catch (IOException e) {
        Log.e(TAG, "IOException There was a problem reading the data: " + e.getMessage());
        return null; 
    }  
    catch(Exception e)
    {
        Log.e(TAG, "Exception An error occurred: " + e.toString());
        return null;
    }
    httpclient.getConnectionManager().shutdown();
    JSONObject resp = null;
    try {
        resp = new JSONObject(result);          
    } catch (JSONException e) {
        resp = null;
        e.printStackTrace();
    }
    
    Log.i(TAG, result.toString());  
    return resp;
}

So this works fine when I use Wifi to connect, but using 3G is fails in 16 - 18 seconds throwing a HttpResponseException.

I cannot believe that it's the DefaultHttpClient running async that is the issue, because then it should throw the error on Wifi as well.

EDIT
The web services are written in C#, and use JSON responses.

As requested here is the logcat, giving the HttpResponseException in 18 seconds, as I said in the original question:

07-22 11:13:04.678: D/ProgressBar(26524): setProgress = 0<br/>
07-22 11:13:04.678: D/ProgressBar(26524): setProgress = 0, fromUser = false<br/>
07-22 11:13:04.678: D/ProgressBar(26524): mProgress = 0mIndeterminate = false, mMin = 0, mMax = 10000<br/>
07-22 11:13:31.968: D/GpsLocation(26524): Got location<br/>
07-22 11:13:32.018: D/WebService(26524): BASE ADDRESS [http://[SERVICEURL]]<br/>
07-22 11:13:32.023: D/ProgressBar(26524): setProgress = 0<br/>
07-22 11:13:32.023: D/ProgressBar(26524): setProgress = 0, fromUser = false<br/>
07-22 11:13:32.023: D/ProgressBar(26524): mProgress = 0mIndeterminate = false, mMin = 0, mMax = 10000<br/>
07-22 11:13:32.048: D/HttpPostWebServiceTask(26524): Posting to [http://[SERVICEURL]/Authentication/Authenticate]<br/>
07-22 11:13:32.053: D/HttpPostWebServiceTask(26524): Starting post data call [http://[SERVICEURL]/Authentication/Authenticate]<br/>
07-22 11:13:47.758: D/dalvikvm(26524): GC_CONCURRENT freed 258K, 17% free 8062K/9671K, paused 22ms+25ms, total 157ms<br/>
**07-22 11:13:50.728: E/HttpPostWebServiceTask(26524): HttpResponseException: [18671] millis. There was a problem communicating: Gateway Timeout**<br/>
07-22 11:13:50.778: D/GOT RESPONSE(26524): NULL RESPONSE FROM SERVER<br/>
07-22 11:13:50.778: E/HttpPostWebServiceTask(26524): Error invoking command: null<br/>
07-22 11:13:50.783: W/System.err(26524): java.lang.NullPointerException<br/>
07-22 11:13:50.793: W/System.err(26524):    at com.app.webservices.WebService.checkValidResponse(WebService.java:82)<br/>
07-22 11:13:50.793: W/System.err(26524):    at com.app.webservices.WebService.access$0(WebService.java:76)<br/>
07-22 11:13:50.793: W/System.err(26524):    at com.app.webservices.WebService$1.execute(WebService.java:120)<br/>
07-22 11:13:50.793: W/System.err(26524):    at com.app.webservices.WebService$1.execute(WebService.java:1)<br/>
07-22 11:13:50.793: W/System.err(26524):    at com.blm.android.webservice.HttpPostWebServiceTask.onPostExecute(HttpPostWebServiceTask.java:226)<br/>
07-22 11:13:50.793: W/System.err(26524):    at com.blm.android.webservice.HttpPostWebServiceTask.onPostExecute(HttpPostWebServiceTask.java:1)<br/>
07-22 11:13:50.798: W/System.err(26524):    at android.os.AsyncTask.finish(AsyncTask.java:631)<br/>
07-22 11:13:50.798: W/System.err(26524):    at android.os.AsyncTask.access$600(AsyncTask.java:177)<br/>
07-22 11:13:50.803: W/System.err(26524):    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)<br/>
07-22 11:13:50.803: W/System.err(26524):    at android.os.Handler.dispatchMessage(Handler.java:99)<br/>
07-22 11:13:50.803: W/System.err(26524):    at android.os.Looper.loop(Looper.java:137)<br/>
07-22 11:13:50.803: W/System.err(26524):    at android.app.ActivityThread.main(ActivityThread.java:4947)<br/>
07-22 11:13:50.803: W/System.err(26524):    at java.lang.reflect.Method.invokeNative(Native Method)<br/>
07-22 11:13:50.803: W/System.err(26524):    at java.lang.reflect.Method.invoke(Method.java:511)<br/>
07-22 11:13:50.803: W/System.err(26524):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1038)<br/>
07-22 11:13:50.808: W/System.err(26524):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:805)<br/>
07-22 11:13:50.808: W/System.err(26524):    at dalvik.system.NativeStart.main(Native Method)<br/>

END EDIT

Is this the way I should go: http://www.vogella.com/articles/AndroidNetworking/article.html
Then how do I post data entities using this method?