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

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.