Well I can reliably make it stop locking the system completely now. I switch to
console with ctrl+alt+f2 as soon as the game locks up and then back as soon as
the ring stall messages start to fill up the screen. When I am back the game is
not frozen anymore, but extremely slow. In fact all opengl related rendering
becomes slow. And I get this message each time I switch to console :
"GPU fault detected 146 0x0904xxxc
VM_CONTEXT1_PROTECTION_FAULT_ADDR 0x00029E48
VM_CONTEXT1_PROTECTION_FAULT_STATUS 0x040xxxxC"

How should I give more information?

If I restart the performance comes back to normal