Any ideas for cause or possible remedies? Got a MSI k9agm2 motherboard in old dell case with amd 3.2 dcore?

proc , pciexspress vid powered with bluestar 650w atx sata.. About 7 or 10 mins after boot shuts down and only restart after unplug ac cable.. Proc doesn't seem hot and two fans on vid card but still shuts down. Any input much appreciated.