Earlier this Summer I had such an issue (with web host proxy caching) after
a site move from an old server to a new server.
The feedback from the SysAdmin at that time was:
"Well your seeing caching the way it is supposed to work. If you do not
want us to cache your site at the perimeter then I can setup you up with
a non caching proxy. Another alternative is to set a non-cache header or
simply set a cookie. Any of the above would work."
My 'problem' was in/with a .php request, and I simply sent my own
"non-cache header" at the top of the script. WFM.