Sorry for coming back to you so late. The freeze at the end of the month occurs, when the game has to do various calculations, which tend to get more demanding over time as cities grow. We put a lot of effort into optimizing this calculations.
As for the vehicle window this would need further investigation. If you want, you could send us your save game so we can have a look at it! You can find our email address in the “About / Urban Games” section.