Anyone know what would cause this, I can drive my 2008 base GP fine for a while... sit idle for a few minutes it will idle around 700ish RPM then suddenly you hear the engine slow down and shake mildly when this happens the rpm drops to about 500 RPM but doesn't cut out.. then it goes right back up like nothing happened... this happens very randomly to me, not sure what causes it to do that... any ideas?