sometimes you get script errors in your browser when you run google ads. It is most likely that you have flash in the ads.
Based on normal broadband speed, there is a very high chance that the full page is rendered even before the flash ad started downloading (as it gets redirected to a 3rd provider which needs to run through certain process before pushing the flash to our site -> then the browser need to call the flash plugin…blar blar). As a result, you will see spacing / layout issues near the place where the ad is. Again, if the flash is in the browser cache, user will not experience the layout issue.
sometimes, it can take a long time to debug the issue and most of the time, it is the fact that the flash or images are too big.