If you have an automatic transmission, the torque converter control
solenoid, which locks up the torque converter when you drive faster
than 40 to 50 mph, is not disengaging at lower speeds. It eventually
disengages when the car is turned off after a while. Until it does,
the car will stall at low speeds. It needs to be replaced, or if you
can't afford to do that, you can have your mechanic disconnect the
electrical connector leading to it so that it never engages. This will
result in slightly lower gas mileage on the highway, and your check
engine light will always turn  on a few minutes after starting the car
each day.

Bob