Cache problem

When user login in by entering the username and password on the Home page under the intro text (Welcome and Signup message) section, and click Login, the Home page blinks once, then nothing happens, until hitting the "F5" key on the keyboard (to refresh the page), then it shows the user is already login.

Try this on 4 different computers, the WPMU site acts the same - after login from Home page, one has to hit F5 key to refresh the page in order to see the logged in status and functions (such as post on activity page).

Sometimes, clicking a inner page link from Home page would show the user is login suddenly on the inner page, sometimes it still show the user is not login on the inner. Even if on the inner page it works, going back to the Home page (and without hitting F5), the Home page still shows the way the user is not login.

Try supperadmin and regular users, same thing.

Background info:
fresh install of WPMU, BuddyPress and Group forum 2 days ago (11/26/2012)
theme: Social.
Plugin: Marketpress, fast secure contact form, Wordpress Video Plugin
Hostgator shared plan
New site, no visitors/traffic yet.

Thank you for reading my long post