The 765M runs at 796 mhz, it has a turbo boost that allows it to reach 900 mhz depending on situations but its normal speed is 796 mhz. So what you are seeing is it run at its normal speed (796) and then sometimes run up to 900mhz when the turbo boost kicks in, the duration of the boost can vary on temperature, power etc
Simply put there is nothing wrong with your GPU