Zynga games are very resource intensive and are best played in a fast light browser, such as Google's Chrome. We have found both IE7 and IE8 rarely load the games, but Mozilla's Firefox works quite well.
Even keeping our Flash Player up to date and using Chrome, we too have loading issues quite often. Using CRTL+F5 does help re-load a fresh page. (Just using F5 reloads a cached page only).
Hopefully this is something that Zynga will soon address.