need help

Your Buick has a lock-up torque converter. GM calls it Torque Converter Clutch, or TCC. Ordinarily, a fluid coupling exists in a torque converter, but with TCC this changes to a more efficient mechanical coupling during certain cruising conditions when not a lot of torque multiplication is required. If the TCC stays locked in a mechanical mode as you decelerate, the engine will stall, much like the engine would stall during deceleration on a manual-shift car if the cluth pedal were not depressed. There is a multitude of possibilities that could be the culprit. First of all, check the fluid level, preferably with the engine-transmission at normal operating temperature, shifter in "Park". Also check the condition of the fluid itself. If it is brownish, black, or has a pungent burned smell to it, changing the fluid and filter may help. Beyond that, there is an array of sensors, a relay, and a solenoid that would be best checked by a competent GM transmission specialist. The converter itself may actually be at fault. None of these are easy to diagnose without expensive and specialized equipment and training/experience.