Why can’t I access a website

I am attempting to access our vendor’s support website. The website is https but my connection will not reach it, I get the error page “Internet Explorer cannot display the webpage”; however, I don’t get this message everytime. It will work for a couple of days, and then starts again. None of their other clients have this issue and their site is not experiencing any issues. I can access it from my Charter ISP from my home; we use EMBARQ here at the office. I can access other HTTPS and HTTP websites as well as FTP sites, etc. Embarq says there is no problem seen when they do traceroute, it hits the webserver. I have exhausted all other reasonable resources. Anybody got any ideas??? HELP!!!!