Massive Video lagFollow

Im having a major lag issue with my pc when i play ffxi if i just turned on the pc the game will run fine for about an hour then after that every 10 or so minutes i get a massive lag that lasts for 10-20 seconds Im running win xp pro,512 mb ram,geforce ti 4600(leadtek ultra td a250),1.8 P4,its a compaq i dont have the model # with me now but ill post when i get home.The video card worked fine in my sony vaio but ever since i installed it in this pc its been lagging on all 3d games....Help me please>

my video drivers are up to date ,i defrag every week my bios was recently flashed,all my drivers are up to date . But i tried disabling the sound and it works like that. IMHO i think i should just install a soundcard im using onboard audio now^^.

I flashed my bios b/c i was having problems with my usb ports,But even before i flashed it i was still having the problem,But i turned off the sound and now the lag is still there but its not as extreme(e.g freezing completely for about a minute) Now my fps drops to about 2-3 fps <)><(>

make sure your bios is set to agp 8x with a 128mb video aperature.