That's a weird one. Sounds like an 02 sensor problem, but since you say you just had work done on the car, it's possible something was reinstalled incorrectly.

Let's hope it's not a mechanical timing problem from the timing belt installation... I know the newer cars will actually smooth the idle back out with crazy ignition timing changes even if the mechanical valve timing is off.

You need to take the car back to the shop that worked on it and have them recheck their work.