Incompetence at Ntt Isuzu Newcastle very High

I have sent my isuzu bakkie at ntt isuzu Newcastle for 2nd service it came back stalling sent it back after several calls complaining about the issue they refused to repair it under warranty the mileage is 51000 and 6 months old and it stay there for 4 weeks I eventually agreed to pay and 7 weeks is not been repaired nobody when will be repaired they said they waiting for head office in Pretoria but nobody is following up