I have almost the identical system. Celery 300a O/C 450 TNT2 (Diamond Viper II), but 128 meg RAM.
First, although this may not effect you, Bh6 boards with TNT2's need the RAM chips in the 3rd slot. Dunno why, just what the company said. As soon as I switched mine, it ran flawless.
Second- I can duplicate your problem everytime unless I blow a house fan right on my computer with the case open. As soon as I shut it off, I get maybe 3 minutes in AH, then lock up.
I also glued a heat sink fan right on the TNT board, it went from hot to the touch to smooth-cool-mean running mombo. Still, without the house fan, everything runs awsome EXCEPT AH. It needs the little extra air flow.
Of course a new case with several good case fans would be best but I'm not messing with it cause now it runs fantastic.