Well one suggestion is that they can refund your money for not knowing what they are doing. A shop that has a sign that says "auto repair" can't say "I don't know". If they don't know, they find out, call other shops, do whatever it takes.
Then you have to find a shop that does know something about cars and they have to embark on a systematic diagnosis, such as reading the car's trouble codes accurately. If no codes show up, they should be looking at components that do not throw codes, like perhaps fuel pump, fuel filter, fuel pressure regulator. Also basic tune up parts should be checked and the fuel itself sampled if necessary for contamination.