Question about oAuth 1.0

[dutchguy76]

I build a restful Json webservice with a simple authentication request, using oAuth 1.0
It works fine but after several requests the server returns "The remote server returned an error: (401) Unauthorized."

I can't find out what is going wrong.

Does someone has a clue?

[dutchguy76]

Extra information:
When it goes wrong, the following line of code
WebResponse webrespon = (WebResponse)WebRequest.Create(string.Format("{0}?{1}&oauth_signature={2}", url, param, signature)).GetResponse();

returns this error:
A first chance exception of type 'System.Net.WebException' occurred in System.dll