Warning sign OIL LEVEL with crazy warning tones
 

Hello everyone,
I am a very happy A3 2009 owner. She is getting old but still fantastic car which I really took good care of. I love the drive, the speed, the look.
But today, I sometime wish to leave her in the ditch and call a taxi.
I have this red warning sign "Stop engine immediately and check oil level" with these crazy 3 warning tones every time the engine goes from 4th speed to 3rd. (1700rpm). Of course oil is perfect and I even put top quality in it. It's getting worst and worst.
I know exactly when it will come out : every time the engine change down speed... and it's THE only warning I ever got on the car.
Were change or done on the car : The oil pressure sensor, the oil level sensor, the wires, the oil pump. I took her to an Audi dealer ($$) they said that the oil pan nut was not an original and that the thread was redone which might do a magnetic zone and nothing can be done (really!!!).... well... not sure about that...
What else can it be ? the computer ?
Please help me someone. This car is a good car but impossible to drive.
Thank you !