GPU / Comp Crash

[dlux22]

I used to run a Nvida GTX650 and was using hashcat-plus and was working fine, although was very slow.

I have now upgraded to ATI Radeon HD6990, but when I run hashcat-plus after about a minute, my screen goes blank/black, although the computer is still running as if the GPU has failed/overheated I think, is there any solution for this, like maybe reduce the usage of the GPU?

Thanks if you can help.