Seems we can isolate the error to your graphics card and your lag component to your insufficient RAM. I would say your Radeon9800SE card is overheating - leading to reductions in performance, particularly with 3D games that require a high amount of rendering. This is both a client side problem and yours. Outspark is unlikely to fix Fiesta's heavy use of resources any time soon, so my best suggestion to you is to
upgrade your graphics card to one that is more heat efficient. While you're at it,
upgrade your RAM too :D.
Alternatively you can find your own creative ways to reduce your graphics card from overheating. e.g. Turning your tower into a fridge for your soft drink. Cool your graphics card and your soft drinks at the same time!
Jokes aside, I've offered you as much help as I can. You don't have to worry about providing any more information, I'm confident it's majorly a problem with your graphics card over heating.