GTA IV lags on updated PC?

I had a MB Asus Gsurf365, 4 GB DDR2 Ram 800 Mhz, an AMD 6000+ dual core CPU and an Ati HD 4850 video card.

I upgraded recently to MB Asus p8z77-v lx, CPU i7-3770, RAM 8 GB DDR3 1600 Mhz, and I kept the same ati HD 4850 video card from Asus.

Now, I used to play GTA IV quite normally on medium settings, and fraps said the FPS was around 18-25 fps, on win7 ultimate 32 bit.

Now with the new system, on Win7 ultimate 64 bit, on the same settings, I got fraps saying 40-50 fps, even 60, but the game is just lagging. It slows down every two seconds. When I am driving a car it goes normally for 2 seconds and the 3rd second it slows down just for one second, and then 2 other second are OK and the 3rd is again slowed down. It's just frustrating. Fraps keeps saying 40-50 fps.

I also noted that my card gets hotter after not using it for games in many months (and I cleaned its cooler). It goes up to 101 degrees celsius. It used to be 80-90, not more. Could it be because of the heat or is anything else that is causing the lag?