@45rpm: I honestly don't know what is stored in those cache files. I don't know enough about the internals of the game. Since deleting them forces the game to newly create several information that gets rid of glitches you have.
Internet cache also stores some info so that you can access a certain page faster, sometimes you might miss new info because of this and you have to refresh your page.
So if a cache file has outdated info or inconsistent data in there that causes the game to glitch, deleting those could very well solve your problem for the moment. But as you say, it obviously eventually comes back so it can't be all in those files.
@AndreeAndree: That is curious. If the outfit is corrupted that should not have anything to do with the thought bubbles. It does seem that there are other reasons for invisible sims than that. I have seen blinking Sims as well that seemed to get reset a lot hence making them appear and disappear very fast (this might also be true for frozen Sims. They just seem frozen, actually they are just reset really fast in some infinite loop).
