BMW Cedar Isle - Service decline

I have made use of BMW Cedar Isle for close on 9 years to attend to the servicing of my vehicle and have always received good service. However, something has now changed. My car has only ever been to BMW Cedar Isle for its maintenance requirements so the full history of the vehicle is with BMW Cedar Isle. My car broke down on the N1 on 24.11.2018 and the AA assisted in towing the car to BMW Cedar Isle. The diagnostics performed on the car only found that an oxygen sensor was faulty. I was charged the diagnostics fee only. I collected the car and later the same day had the same problem with it. I managed to get it back to Cedar Isle who kept the car and told me to collect it again as they could not find anything wrong with it. On Monday 03.12.2018, the car again broke down and was again towed into BMW Cedar Isle by the AA. To date, 7th December 2018, I am still none the wiser as to what the problem on the car is and apparently the technician assigned to the car is not available to speak to me. I went to the dealership this morning and spoke to the service consultant (who is very helpful), but he too has not been informed as to what the issue with the car is, or if in fact, they have found the problem. This is now becoming severely irritating and being without a vehicle is a problem as I am now having to hire a vehicle to get to and from work.