On my way home today I noticed the car trembled slightly when idling. It did it again a few blocks down at another red light and I caught the odometer bobbing: it'd drop about 100-125rpm (roughly 700rpm) and then go up to about 900-950 before stabilizing around 850. It would do this intermittently the entire drive home. Any ideas what this might be? Also, my radio locked out. I turned the car on and it asked for my code, which is a first since initially inputting it. Thanks, guys.