[Don Plumley]

While it does not idle like a Lexus, if it's enough to shake the car, it's worth tracking down. If you are not throwing a random misfire CEL (P0300) or bad sensor CEL's (like O2 sensor), then you have a tough debugging problem. The range of things that could cause this include (with credit to Steve Weiner):

O2 Sensor, Wiring
Vacuum Leak
MAF dirty
Dirty Injectors
Ignition: Wires/Caps/Rotors/Plugs worn
Various Hall sensor issues
Failing Dual Mass Flywheel
and rarely, wearing valve guides.

First things first, consider running some Techron and clean the MAF using MAF cleaner spray. At 12K miles, but 10 years old, have you done any of the 60K major service items?