The car stalls while driving and takes 15-20 min. to start

It appears you may have a bad ignition coil that cuts out after it gets hot. Not uncommon.

Could be crankshaft sensor,take too long to show a code.

Check fuel pressure may be fuel filter or pump.

First you add octane booster to two year old gas. It is too late for stabil. Find the idle air solenoid and take it out and clean it with throttle body cleaner. Try not to push it in or out. Reinstall and turn on key for I min. and off for 1 min. then start the car.

No codes would point to fuel problem such as low pressure.
Was the car left overnight at the shop so it would be a cold start for the mechanic ?

Hello and welcome to FixYa!
I'm suspecting that you have a faulty ignition coil. If the coil gets hot it tends to shut down the car. Once cold it will fire right back. I suggest that you have the coil checked by a mechanic and replace if necessary.
Hope this helps and thank you for using FixYa! Have a good one!

you may not have the fuel lines sealed, they can **** in air without leaking fuel, especially in the tank, whre the pump connects

Are your battery cable connections tight and secure. When was the last time the fuel filter was replaced???

a bad crank sensor can cause this. shuts off while going down road, and will start about 10 min, later with no problems runs fine until it quits again. hope this helps,

There's a possibility that your fuel pump is going out. What's happening is that you can drive for the 45 min. and the pump gets hot and shuts itself off. After it cools off, it starts working again.