Thanks for the ideas but in the end it seems to be the temperature of my computer case causing the GPU to start to over heat. What I did not think about was that I have just moved into a new house and it did not occur to me that my new place might be warmer (which it is) all so it has been hot for last 2 days and it has now become cooler and I have had no problems at 3.4Ghz.
Thanks for your replies I am glade it was nothing complicated in the end.
xSTLx I have to say that I think it is a bit of a old wives tale that a over clocked CPUs or GPUs will shorten there life span as most CPUs or GPUs are just under clocked versions of there bigger brothers e.g. NVIDIA 6800 GT is just a under clocked 6800 GT Ultra. Thanks again for the help tho ^o^/