Do what Terrister suggested - together with closely inspecting all components in your case. Reseat all connections and make sure they are firmly plugged in. Reseat the video card.
It sounds like it could be a memory issue - download memtest86 from
www.memtest86.com and let it run for at least 10 loops or overnight. See if it reports any errors.
Another thing coming to mind is the power supply. It seems there is a generic 400W in the case you have. Is this correct ? Could you read the model name/ number and its specifications off the side or provide a screenshot ?
You mentioned MBM5 - are there any voltage anomalies around the time the crashes happen ?