Sorry, I should've mentioned I don't know what might help you, and was just giving some general advice.

FWIW, I've heard of some versions of IE caching pages including headers, leading to strange behaviour with cookies. To see if you suffer from caching woes, you can try the following snippet which I've filed away under overthetop_anticache.pl:

use POSIX qw(strftime);

print header(
    # date in the past
    -expires       => 'Sat, 26 Jul 1997 05:00:00 GMT',
    # always modified
    -Last_Modified => strftime('%a, %d %b %Y %H:%M:%S GMT', gmtime),
    # HTTP/1.0
    -Pragma        => 'no-cache',
    # HTTP/1.1
    -Cache_Control => join(', ', qw(
        no-store
        no-cache
        must-revalidate
        post-check=0
        pre-check=0
    )),
);
[download]

Update: fixed date as per davis.

Makeshifts last the longest.