Why does the computer reboot when the video is loaded?

Question

Video cards

Here is a strange thing:
there are two system
units 1)Asus m4a88t-v evo
amd 955 BE (125w)
radeon 6870 DC(120w)
chiftec 550w
2)asrock 980DE3/U3S3
fx 6300(95w)
radeon 6870 DC(120w) ocz
500w
the same. The computer is rebooted when the video recorder is loaded with a donut (on the first system unit at 78 degrees, on the second at 73 degrees).
Where to dig?
The power of the BP should be enough.
I think the northbridge is overheating. It doesn't feel very hot (50-60 degrees)