Black Ops II PC Fps down in some areas

Hello, I'm having problems with Call Of Duty: Black Ops II pc
PC Specs:
GT430 1024MB 128bits
Intel Core 2 Duo E8400@3.00Ghz 3.00Ghz
8GB RAM
Seagate 500gb HD
Real Power 500w

With the settings on LOW, in some areas with 40-50-30 FPS, CoD players know that 30-40-50 FPS is horrible in all maps have a point that the FPS is low, but the FPS is that I can usually 80-90-100 FPS (also in MW3, MW2, CoD4), walking elsewhere. I wonder why the hell you 40-30 FPS in some areas of the game, these areas are not open, even indoors, if I leave it (60-70-80) if I go (40-50), on the map "Express "for example, if I climb on top of the train i got 40 FPS, if I got him down to 60-70 FPS?!?.'ve reinstalled the game many times, I have updated the graphics drivers from Nvidia 310.90 at the time. VIA Sound Driver for Windows 8 also updated, tried everything 800x600 LOW OFF all and nothing, I can only FPS in most areas that do not have problems, but when I walk in areas that have problems when the FPS drops is the same coisa.me help what I do, I'm having this since 27/11/12 if I am already prestige master with this problem KD 2.07.I'm appreciate other solution for this, already tried support for 2 months and they just answer me useless things.