Grand Theft Auto IV ATi 4870

Hi i no its a pretty old game now but i cant get it to work on my rig, before i start PLEASE do not advice me to update my rig thank you.

AMD Athlon II x4 630 - 2.5GHZ
6GB DDR3 1600Mhz
3TB SAMSUNG
Gigabyte GA-890XA-UD3
WINDOWS XP X64 - I NO OLD BUT I LIKE IT
Novatech PowerStation Black Edition 1000W Power Supply
ATI RADION 4870 - 512MB SAPHIRE


right here's the problem when i first install it, it runs fine for about 5 minutes then out of the blue crashes to the desktop with the error "Fatal error out of video memory please restart your pc" Now it doesn't matter what settings i do weather that be high or low its still the same, now after the first try after the crash i go to start the game and during the loading screen it will crash then so cant even get into the game now, i have tried all the commands -memrestrict and all of that stuff with no galore, i have uninstalled my gpu drivers with drivers sweeper to and done clean installations on countless versions of CCC tried all the gta patches form factory set up to 1.0.7 and still the same, i have been trying to fix this for 4 days now can someone please help me out its ****** me of now.


PS my gpu is 512 i no but ive played gta 4 fine on it before in my older system with the same card thanks!