One of my sapphire 5830's does this, the reason being too low fan speed. Core stays at a stable 55-60c but the VRMs are overheating due to a lack of airflow to them (fan speed is about 35%). It crashes when the VRM gets too hot.
Also, even relatively low overclocks or voltage increases can cause these cards to crash after a few hours.
Crappy design but you get what you pay for....
Thanks for your reply
It always been on;
- Core clock 900
- Memory clock 500
- Fan speed 90%
Should i put it back on stock?