Frame-rate Issues

[USA Driver]

Anyone else get this? This is on US 101, between the outskirts of San Francisco to the Golden Gate Bridge exit.
Around where the SF Intl Airport is and all of that, I can feel my FPS dropping. But usually it's still playable.

However, it was ALOT worse yesterday when I was streaming to a friend, and it was raining. Now I got a pretty decent computer, but I was getting as low as 2-4 FPS in that area. Nearly unplayable.

I just want to know why it's so bad. I mean, you got cities like LA and Reno, and it never studders there.

Stats;
OS: Windows 7 x64
CPU: AMD Phenom x4 9550
Video: ASUS Geforce GTS 250
Audio: Onboard (High Definition Audio Device)
Memory: 4GB DDR-2