When start fd1 i play fine for about like 5-10 mins.

 

then i get a horrible lag, so bad that its unplayable.

 

how do i fix this?

Bump, need answer fast.

check if your card overheats while playing or cpu.

Its not a cpu issue.

 

how do i check if it overheats?

Try Speccy. You can also see it if your CPU gonna overheat.

cpuid or cpu-z Both read your computers cpu and graphic cards temperature also tells you the airflow.

 

http://www.cpuid.com/

 

Also try to stress it with furmark while you monitor your temperatures, This way you get a idea of what your temps are when running the game.

 

http://www.geeks3d.com/20120510/furmark-1-10-1-gpu-burn-in-stress-test-videocard-geforce-radeon/

Check your temperatures for both your GPU and CPU.

 

You can check them with Speccy, Realtemp (CPU only), GPU-Z (GPU only), etc.

 

To check whether your CPU is throttling or not; use CPU-Z.