I had the same problem a couple of years ago and discovered that the core on the rad was blocked,even tho the car was put in to an RR dealer. They said head gasket had gone so duly told them to do both head gaskets, week later went to pick the car up all seems ok. Drove out of there to be 3 miles from home when again it blew all the water out, so took it back for them to discover that it was the rad core. It was stone cold in the middle but hot on the out sides, they changed the rad and no more problems and I also got my money back from there diagnostics telling me the head gasket had gone from the hydrocarbon test. So check the rad first, run the engine till warm them try the rad.