Timing Problem?

I have a 2002 Sebring LX 2.4 DOHC automatic and just had the timining belt replaced, it seems like the engine is fine (sounds fine at idle), however, when either in D or R at idle (after coming to a stop), if the brake is released, the car seems to push itself on and off, kind of like jackrabbit start with a stick shift. The mechanic advises that tranny and timing are fine, how can I confirm my timing MAY be off or if there is another problem...I have 100K miles on it...my gut tells me its the timing belt which is installed but the timing may not be completely set properly, which is causing the engine not to be stable while in gear, causing the herky jerky motion at idle...after the gas is pressed, it works fine, but still sluggish around 2-3K RPM...

Thanks for any reply...

Carlos