No one can tell me what the problem is!

I bought this car through a private sale 4 months ago and driven it successfully for 3 days in total. I replaced the battery with a new one and done all the basic engine checks/top ups.

The car turns on, then after 5-10 mins of either being in Park or Drive, the car switches off. It usually turns back on successfully after a few minutes, however it can stall while turning on at times (not a manual just FYI).

I had it towed to my mechanic. They said they couldn't see any codes or faults and recommended it go to the transmission specialists (assuming a gear box issue).

The transmission specialist tried to pressure me into a new gearbox (quoting me almost how much I paid for the car) but after looking at the problem himself he found no issue either saying its not a gearbox issue. I drove it home and the car had the same fault the very next day.

I then took it to Hyundai Service Centre for a full diagnostic check and they also cleared the car as having no issues! They did however recommend replacing the "crank sensor" but couldn't guarantee that would solve the issue.

As a woman who lives by herself I'm really not sure what to do now. I can't even manage to sell the car for a few thousand less as understandably no one wants to buy a car with an issue.

ANY advice would be appreciated!