Check the basics Monotan...
Anything running in the background? You playing audio? You downloading anything?
Does it do this lag on any other games?
You said video lag - is this a video played during the game or is this gaming lag?
Are you overclocking the card?
If the obvious stuff is fine - whats the temp of the 4870? Run GPUz during the game and check the temps. A 4870 can run hot. Is the fan pretending to be a whole house vacuum cleaner? Do you set the fan to a higher speed because you need a whole house vacuum?