Bought a defective car

2015/05/14 I bought an Audi A3 20tfsi from Mastercars.Being a 2007 model with 135000km on the clock was risky, but felt confident with a 80 point check done, roadworthy and I took a 2 year warranty.2 weeks after the car had problems.It drives normal, jerks, loose power and drives normal again.Only happens once in two weeks and is very difficult to point out to a mechanic.They took the car in and couldn't find anything wrong. Said I must take it to McCarthy Audi Menlyn. No fault found again. Got an extra opinion from Audi Center Centurion.They had it for a whole week and R16000 bill to get it fixed.Tried to claim it from the warranty and they declined the claim due to being wear and tear.Dealer said I must bring it back.No fault found again. It also had no oil a day after driving to work. A while after the car had a serious loss of power and jerked for 5km non-stop.I consulted a private mechanic and said it just needs a serious service.The problem persisted, but not using oil anymore after service. The dealer refuse to refund me. I'm also loosing amount paid for the warranty and my deposit.Car has smaller issues like broken elec window, wheel&steering noise. Very heavy on fuel!