My Mustang built in June 2016, RHD for Australia. On the last 100 km of a 6000 km round trip the engine warning light came on partially, then off & finally full on. The engine was running rough as though it had lost spark on one or more cylinders. I was cruising at highway speed of 110 kmh (2000 rpm) and about 80 km from home. Once off the highway the engine ran rougher at lower 1000 rpm. Three days later, 1 April, I drove it to the dealer for an arranged repair, it is still there. The engine would not idle below about 1000 rpm and it was a beast to drive in slow traffic requiring Neutral and revs at traffic lights to keep it running. On day one I was told that it needed a new head gasket before proper diagnosis of the issue. After a week I was told that a valve spring was broken. The following the service manager told me that they had removed both heads but he did not have details of the problem. It is due for completion tomorrow. Has anybody had a similar issue or thoughts on what could be the problem. I am not expecting straight answer from the dealer ! The total traveled is 36,000 km without prior issues. I enjoy driving the car and was very pleased with it - until now.