flash animation lags

Hi,

I currently made a flash animation. the background is the only one that is animated and scrolls to the right. But my problem is why it lags? Is there any way to not make it lag?

here's a link of the swf file you can view:

http://enciel.awardspace.com/bus.swf

I tried it on a high spec pc and it does not lag when it is in normal screen(640x480) but when I view it in full screen it lags. There are no scripts or anything in this file just animation.

anyone? does it lag when you view it too??