Hello, my name is Mark and I will try help you with this.
The first thing you need is a tool called a flight data recorder.
This plugs into the EOBD port and records all the engine data as you drive normally.
After the fault occurs you connect the tool to your computer and it will display the fault code plus all the engine sensor data as the fault occurred.
You can then use this information to diagnose the fault.
The above tool is a clone of a dearer version.
It is about £70 - £80 cheaper than the licensed version but should still help identify the fault.
That's not true. What I am telling you is that to find the fault you require this tool as the garage you are using cannot find the fault code whenever you take the car to them.
Your other option is to take the car to them as soon as the light comes on, don't switch the engine off and see if their scan tool can find the fault.
A lot of garages also carry this tool especially for this type of situation.
You might want to try another garage.
Your question asked "Please help if you can". There is no way I can diagnose the fault when even your garage can't. All I can do is advise how to find the fault.