Problem logging on to Navision Web service from Windows mobile\.net compact

I have a problem logging in to web services from windows mobile, i.e. .net compact.

The problem is when I log in, it SOMETIMES gives me invalid userID and Password error, despite the fact I know them to be correct.

Being Window Mobile, i.e. .net Compact 3.5 I cannot use functions like usedefaultcredentials, so use the following:

POService =

new SG1.PVPurchOrder.DynamicsNAVPVPurchOrderWMS_Service();

POService.Credentials = 

newNetworkCredential(this.sUserName, this.sPassword, this .sDomain.Trim());

When I come to read a PO I know is there I use :

this.poSelectedPO = this.POService.Read(txtPONumber.Text.Trim());

but it SOMETIMES gives me invalid userid as error.

What am I doing wrong?