Just Cause 2 lag in towns

[slovenc95]

I have a problem with lag in Just Cause 2. It only lags in towns. I usually get 60 fps in other areas but when I'm in a town it drops to like 40 fps and it's noticeable. I'm not playing on max but I think that my PC is enough powerful for atleast high settings. Btw, can someone tell me how to check the GPU and CPU usage while I'm playing?

My settings in game:
http://shrani.si/f/42/tS/m334q5L/zajeta-slika.png

Intel Core2 Quad Q8400 3,2 GHz
Deep Cool ICEEDGE 400 NI
4 GB of RAM DDR2, 800Mhz
Sapphire AMD Radeon HD 6870, 1 GB, 940/1185 Mhz
Corsair 600CX ATX 600W
ASRock G31M-S
Case Fan Led AK-170CB-4BLS 80 mm
Windows 7 Ultimate, 32-bit
LG 23,6" LCD W2443T-PF
5.1 Microlab FC 360

Thanks.