1991 Camaro rs 3.1L 218000 miles

91 Camaro, 218000 miles, 3.1 L

When I start the car it revs up to about 2500 rpm, and than takes a while to settle down. While driving at low speed the rpm`s surge high and low until it dies. We brought it to Pep Boys they did a engine diagnostic test and it was ok. They cleaned the fuel injectors and checked the vacuum hoses. The car has still the same problem