It sounds to me like the memory on your Flash cache needs to be cleared. BW is a hugely intense game when it comes to graphics, which is why they recommend downloading it. It isn't like any other game for this reason, there's too much info in this game for the Flash Player to handle to without being cleared sometimes. It makes no difference which browser you use, or anything about your computer, it's strictly a Flash Player issue. I clear my Flash cache on the opening screen of BW about once a week.

I'm not at all comfortable with telling anybody to do anything to their computer. I need help with that stuff most of the time myself. So, I'm going to just refer you to this thread that explains how to clear the Flash.

http://forum.djartsgames.ca/?thread_id=971

The ccleaner program that Monica mentions is a great one too. I use that as well, though not as often, but as often as I would normally do a complete clean-up. That cleaner is not Flash specific, but does a cleaning-up on your whole computer.