HTTPS error page cannot be displayed

Hi
i have a user who is having problems accessing a https site. The home page (westpac.com.au)is fine, but when trying to access the online page it gives “page cannot be displayed” and further down the page it says “Cannot find server or DNS error”. Sometimes it comes on without issues and other times it does. Why is it being intermittent. Is it an internal DNS issue. I’ve tried adding the page to Trusted site in my IE but problem is still there. Any help is much appreciated.