Powersupply : gigabyte 750 watt
gigabyte don't make power supply
what is this v-sync ?
V-sync is a proceadure the VGA may use in order to synchronize the horizontal and vertical refresh rates, thus avoiding image tearing. Some people do not think it worth the performance impact, while others, like me, consider it mandatory.
You said you think your card is not working properly because it is running COD4 at 120+ FPS, and you expected 250+. Don't get me wrong, but you should be glad your card is not wasting resources generating frames you won't see. There is nothing wrong with it.
If it is lagging, it is surely not running at 120FPS.