proxy - random Authorization error 407 using libcurl

Question

i am using libcurl to do HTTP request.

The request goes through authenticated proxy.

Below are my settings with regards to proxy

curlResultCode = curl_easy_setopt(curlHandle, CURLOPT_PROXY, <proxy-server>);
curlResultCode = curl_easy_setopt(curlHandle, CURLOPT_PROXYPORT, <port number>);

//curlResultCode = curl_easy_setopt(curlHandle, CURLOPT_PROXYAUTH, CURLAUTH_ANY);

OR

curlResultCode = curl_easy_setopt(curlHandle, CURLOPT_PROXYAUTH, CURLAUTH_NTLM);

I cannot give user name and password for the proxy as i expect CURL to intelligently find it like how "winhttp" interface does.

I have tried both ANY and NTLM. However, at times the request goes through successfully through the proxy. At times i am getting "Authentication required: 407" during the easy perform.

Can someone tell me what i have done wrong here to see this random behavior?

Thanks for reading!!!