1999 Pontiac Grand Am Shifting Gears is jerky

When I drive about 100 miles or more at a time the service engine soon light comes on and the next stop light or stop sign I come to the car stalls. After the light comes on the car jerks a lot between shifting and when I go from reverse to forward and visa versa. When the needle gets below 1RPM it jumps around a lot and I can feel the motor of the car wanting to excel then stall and so forth. I'm not sure if this is a problem with the transmission or something else. I do a lot of inner city driving where the light never comes on and this doesn't happen. Please help me!