IE6 and IE7 bug for Proxy NTLM authentication

We are using Squid as a proxy with NTLM authentication. There is a web site send 10K-100K POST requests for each page load. The IE6 and IE7 intermittently have the difficulties to load these pages. The further investigation shows some misbehavior of the IE. When the browser received “407 authentication Required” from the proxy, they should always send the request again. But that is not always happening in IE6 and IE7.