system crsah after 5 mins of play

[Goth]

I think its down to the graphics card Radeon HD4870 , it almost seems like the card runs out of memory after 5 minutes and gives up the ghost, yesterdays hotfix has installed and the game runs much smoother(was laggy in battle mode before) but now runs very smooth, however the game is still rebooting my stsem after 5 mins. I have down graded the graphics but each time I reload the game it has ignored my changes and reverted to Ultra setting. Its a bit Wieeeeeeerd!!!!
Goth