Desktops: why does my pc lock up?

why does my pc lock up?

my pc tends to lock up durring game play. some times the pc will run games (oblivion, Deus, KOTOR etc) for hours with out problem. other times it locks up within five minutes requiring me to do a hard shut down. I started oblivion in windowed mode to monitor the cpu and ram usage and neither one of them maxed out.

i have.

Win Xp home
p4 2.8ghz
533 fsb
512 ddr
radeon x1600 pro 256mb
Phillips sonic edge 5.1 sound card
powerlink 400 watt psu

i have MSI core center and this is how it looks right now.

vcore =1.55v
3.3v=2.99
+5v=4.6
+12=11.66
CPU temp=25c
sys temp=28c
cpu fan 2556 rpm

i have ran hotcpu tester pro, and 3d mark 05. they both seemed okay. I have all the updated drivers etc.

any suggestions?

thank you.