Sabertooth X58 random shutdowns

Just recently my computer started to shutdown randomly. Sometimes after
several hours and sometimes after a couple of minutes.
After several weeks of this I didn't use it for a week or two. After turning
on again, the monitor said no video. Today I replaced the
video card and same message. Replaced the monitor and got video back. Booted
up and everything came up for about 5-10 min.
and then shut down again. The unit is 16 months old, with an Intel i7 -960
and 3 sticks of 2G dominator memory. I don't know what to
do next..
Appreciate any help to resolve this issue.

Guy