After an hour or so some of the characters stop walking so I have to quit, and restart my computer. There are two other weird things: i can't remove one of the keys from the dungeon (from Esme's room), and sometimes when I save it doesn't work and I loose that save (it says it saved, but it doesn't and then asks if I want to quit the game or continue. I'm saving a lot now under lots of different names which is a pain since it takes so long to save the game.
The issue with characters suddenly failing to move occurs in versions of Mac OS X past 10.3. There is no workaround for it; you have to quit and relaunch the game to continue playing, sorry.
The issue with saving is caused by Spotlight indexing the temporary data at the time the game tries to delete it (the game quickly copies data to the tempsave folder when building the save file and tries to delete it almost immediately; the way it does this can fail if Spotlight is still indexing those files). You can add the entire Baldur's Gate II directory to the Spotlight exclusion list (in the Spotlight pane in System Preferences) to solve this.
You can also ensure that your temp folder is actually a folder and not a file (some versions of the Mac port will create temp as a file, which will affect the game's behavior and cause saving the game to fail). If it is a file (it's in your Baldur's Gate II folder), you can delete the file and create a new "temp" folder in its place.
Any idea as to what is going on? I don't know if it has anything to do with it, but I should mention that I used the cheat where I kept Imoen by using the console to exit the dungeon. Could that mess up the game? At least this time the circus quest worked correctly and I was able to do the fight in the Den of 7 vales.
Likely not, but this is the reason you have that undroppable key still in your inventory (correctly taking the dungeon exit triggers a script that cleans up some of the opening dungeon items and starts the battle cutscene for first entering the Promenade; these things won't fire if you cheat past it).
Your initial assertion error was caused by an incorrect effects index in an item or spell introduced by one of the mods (we shouldn't have any more of these in the fixpack, I hope). I think the trouble you had with the circus tent may have been caused by UB (certainly, we shouldn't change enough of it in the fixpack to have it fail for you), since it touches the sequence to add some quest for Kalah or something.
If you search around, you should find links to a mod you can run after installing all your normal mods that will scan all your items and spells and correct the effect indexes so that that particular assertion will never come up.