You have given little info about car, model, size engine, etc.
What happens when you try to start it after shut down? What do you hear? Do you smell gas? Is the car hot or cold when stalling. At a stop or when on the highway?
1) Firstly... suggest to read all blogs and you'll begin to see a pattern. Take notes. Hone in on answers related to your problem.
2) without codes, I'm surprised the dealer didn't say something about gas related issues or security system... which pops up first on the hit lists... gas filter, gas pump, gas pressure, alarm system, key system, locks malfunctioning. fuel injectors clogged.
3) sensors... about 7 of them... (2)02 sensors, map sensors
4) catalytic converter clogged
Don't trust dealers word... check for recalls yourself. I found 2 for my car and was never contacted by manufacturer... one was reprogramming the PCM to the catalytic converter and the other was a transmission issue, where car sometimes didn't adjust the gas mixture higher when coming to a stop and shifting to first.
Any way... good luck and read the blogs