beginning of every game lag (stuttering)

Hi there,

I have a big problem, and it's made the game 100% unplayable.

I'm using a HD 7790 (latest beta drivers)

Amd athlon ii x3 450 (old, but cpu ran bo2 absolutely fine and its the same requirements for ghosts)

HDD: 500GB

PSU: 500W corsair

So every single time I start a private match, or multiplayer match, the first minute or even 30-40 seconds of the game, I will lag all over the place and after the time has passed my game runs completely smooth?

I was just wondering what the hell is up with this?

ps:graphical settings don't make any difference, tried running all low, all maxed, all med etc..

If I could have any assistance would be greatly appreciated.