Intermittant Low Idle
 

I have some focuses in my rental pool, as they are pretty bullet proof cars. But, one of my cars, a 2005 SE with 24,000 miles on it, has a problem with low idle at times. I've taken it to a mechanic, but they can't find anything. I can hear a vacuum assist of some sort kicking in when this happens, but that just makes the idle speed vary from slow to regular about every second or two. If I turn the car off and start it again the problem goes away and it runs fine the rest of the day. Any ideas?