First two things that come to mind, and I know you said you ruled out vacuume leak, but did you or the previous owner try spraying brake cleaner on the intake boot? I'm talking about the boot just behind the AFM. This boot often cracks and the tears are not always visable. Spraying brake cleaner on it while the engine is running will cause the idle to fluctuate if there is a leak.
The other thing I'd look into would be the CCV. It's a black disc located on the driver's side of the engine compartment with a little slit on the bottom. Sometimes this valve gets clogged up and starts to hiss. If you could hear it hissing at idle, it's probably bad. There's no way that I know of to check it without destroying it though. The plastic is usually so brittle that it falls apart in your hand when you take it apart. And finally, another thing that can cause your symptoms would be a clogged crank case vent. Not the actual valve that I was just describing, I'm talking about the hole towards the back of the valve cover where the hose connects. Sometimes this hole gets clogged with gunk and causes your crank case to overpressurize. Here's a pic of your engine bay: |