Tach Dancing

Okay, so heeded the request to search before posting a repeat question and I did come across a good bit of information but nothing that I haven't already checked or had ruled out. So, now at my whits end I emplore the aid of 2pros.

I have an 01 Altima and am experiencing the inexplicable idle shudder. When coming to idle, the tach starts dancing between 500 and 600 rpm. If I kick it up to 1200, the running is seamingly smooth, and drops back down when released.

Items checked:

[u:29040ded96]EGR Valve[/u:29040ded96] - Fine
[u:29040ded96]Plug Wires[/u:29040ded96] - All resistance =
[u:29040ded96]Plugs[/u:29040ded96] - All 4 good burn, not ashy/not fouled
[u:29040ded96]IAC[/u:29040ded96] - Cleaned and functional
[u:29040ded96]Intake manifold[/u:29040ded96] - Sealed, no Vacuum leak
[u:29040ded96]Various Vacuum points[/u:29040ded96] - No detected leaks
[u:29040ded96]Fuel filter[/u:29040ded96] - new, though would be a little more sporadic than just at idle so wasn't really suspect.
[u:29040ded96]Injectors[/u:29040ded96] - All resistance and running voltage = and normal.

Codes Received:

0140 - Lean Condition Detected
? - Cylinder 4 Misfire
? - Multiple Random Misfire
? - Knock Sensor

? = Not sure what the code actually came up as by number.

My brother is attending UTI in Houston, and I had him take the car to class and they ran tests on it but (with the help of instructors) were unable tio determine the problem.

I'm basicly at an end point to my patience with this one. Looks like I'm gonna be putting it in to the dealership unless someone has some other suggestions.