No guarantees, but from the sound of it, the most likely culprit is browser cache. If you have a good, high-speed, connection, you want your cache set relatively small. In Firefox, I keep mine at 5mb where it wants to default to 300+
If your connection is slower, a larger cache may help, as the graphics can be stored locally instead of being fetched over the network. If you use the browser a lot for other things, it's possible that the various art for your city is being flushed and has to reload each time you start on a given day.