Mine used to do this, too. Finally the check engine light came on and I took it in for service. This was earlier this year. Don't remember exactly what they said was the cause, but they did end up fixing several major things, including a new water pump, intake manifold gasket, thermostat, and timing chain tensioner. It doesn't shake or feel like it's going to die anymore while idling. FYI, the intake manifold gasket cost $90 plus $160 in labor.