I have similar problem.
After about 30 mins of multi play on my PC every car is freezed on the truck. Cant see anything. Other people tell me that im lagging even 600sec. I dont thnk its a connection/network problem, because on my laptop on the same net everythink is ok. I tried to use 2 OS: Win XP home 32 bit and VISTA 64, it was the same.
My PC is:
GA-EP45-DS3 with Intel core duo processor;
4 GB RAM;
2x ATI Radeon 4850 cards;
logitech momo wheel.