hey
I had that exact problem, ten minutes with doom3, it would lock for a second, then doom3 would crash...and i have a 2.8 HT with 512MB of ram.
solution
ati's 4.8 or 4.9 catalyst drivers...Lemme see if i can find a link..
found it
http://www.atitech.ca/products/catalyst/index.html
After this, doom3 worked like a charm and i just beat it last night....god is wonderful