I built a similar system to yourself in March and had similar problems, it'd boot me out of games after a minute or two, namely BF 1942 and Deus Ex.
The build: A7N8X Deluxe, 1gb RAM, Radeon 9700 Pro, Win XP (SP1) etc etc
It turned out the problem was down to the RAM, the A7N8X is very fussy and prefers Corsair. (Scan confirmed this when i called them asking to exchange the memory, "you should have been advised to buy Corsair with this at time of original purchase as it's a known problem). I changed the original RAM i had to two sticks of Corsair Platinum and everything worked fine.
This may not be the cause in your case as i see you have twinned RAM sticks, but as your problem is exactly the same as mine maybe it is. Try swapping RAM if you know anyone who has some you could borrow to test.
Also, try click here They have a forum under the Asus section specifically for the A7N8X and is an excellent source of help and information.