the engine shut off automatically during deriving,

Perhaps you should try integrating instead. =o) All seriousness aside...

It sounds like the torque converter lock up (or TCC) solenoid has gone bad. Take it to a transmission shop and have them check it out.

-Bruce

Could it be a bad injecting system? because sometimes it happens when I just got off highway and the car still moving on a local road.

This sounds very much like a problem I had in an earlier model Buick. Driving on the freeway system, it was usually okay, but when I would let off the accelerator to exit, it would sometimes die. (And that is a real shock when you have no power steering, no brakes, etc)

Several trips to the dealership (the car was still under warranty) proved to me that the shop was incompetent. They could never reproduce the problem, and I asked to ride with the mechanic to show him what was happening. Never happened.

I contacted Buick directly, and they knew immediately what it was...They had had a bad run of semiconductors in that particular ECM series.. Seems our dealership shop crew never read the TSB's.

I suggest you check TSB's on yours. It might be a similar ECM problem, but there could be other answers as well.