my 2003 chevy impala stalls out in traffic on 2003 Chevrolet Impala

The car starts and seems to operate early in the day and on highways. It stalls out in traffic. We've had numerous so called "mechanics" who've diagnosed our car and tried, unsuccessfully, to fix it. My wife and I are retired and rely on this vehicle as our main source of transportation. Can you help?

Asked by for the 2003 Chevrolet Impala