I had the same problem, called Apple Support thanks to workindog's post, and they fixed it. Here's the instructions for cleaning out the caches etc. Be careful:
DO NOT DELETE THE FOLDERS THEMSELVES!!!! OTHERWISE YOU'LL HAVE TO REINSTALL THE OPERATING SYSTEM
go to Library > Caches
Select everything IN THE CACHES FOLDER and drag to Trash, BUT DO NOT EMPTY THE TRASH, AND DO NOT DELETE THE CACHES FOLDER ITSELF!!!!
go to Library > Launch Agents
Select everything IN THE LAUNCH AGENTS FOLDER and drag to Trash, BUT DO NOT EMPTY THE TRASH, AND DO NOT DELETE THE LAUNCHAGENTS FOLDER ITSELF!!!!
go go Library > Saved Application State
Select everything IN THE SAVED APPLICATION STATE FOLDER and drag to Trash, BUT DO NOT EMPTY THE TRASH, AND DO NOT DELETE THE SAVED APPLICATION STATE FOLDER ITSELF!!!!
THEN RESTART THE COMPUTER
THEN, AND ONLY THEN, EMPTY THE TRASH, BUT MAKE SURE YOU DIDN'T DRAG THE FOLDERS THEMSELVES INTO THE TRASH BY MISTAKE
She said this problem is caused by too often putting the macbook air to sleep instead of shutting it down and restarting. So you may also solve the problem by shutting down and restarting. If you follow these instructions, though, it speeds up the computer quite a lot, so if you use it intensively, then maybe do this every six months or so, just be careful about the folders so you don't stuff up the operating system