if u ppl r hardcore gaming experts i need some help badly please!!!!I can run fear,hl2 but i always get this chop lag...

User Rating: 9.1 | Half-Life 2 PC
Ok dont assume i have a pos puter cuz i dont its pretty dam n good.I have

amd athlon 64 3500
2.5 gigs of ram
7800 bfg gt 256mb
audigy sound card
Asus a8n-sli mobo

so if u can see i have an amazing machine its just i can max on prey,doom3,bf2,any game but fear,and hl2.the strange thing in fear is it runs but freezes alot.Half life 2 hits 120-140 fps but does this strange lag spikes all the time where u run smooth then a small lag like that game is catching up.Im hoping instead of spending money this is what everyone is getting cuz its not my hardware.or u ppl r fim about OS cuz i think the copy of XP sp1 running 32 bit bring my down to 32bit cuz i have a 64 bit cessor and just xp 64????