Sessions....... :(

[eluser]Developer13[/eluser]
I came across what I believe to be is a similar issue to yours with one of my applications... but it would only happen on a specific server and sometimes the problem would exist and other times it wouldn't (don't ask -- I don't know!).

Anyhow, I would recommend setting the following in your system/application/config/config.php:

$config['cookie_prefix'] = "http://www";
$config['cookie_domain'] = ".domainnamehere.com";
$config['cookie_path'] = "/";

That might solve your problem.