gpu bus problem

Hello, I have one question. When I play Assassin's creed Odyssey as it runs "smoothly" with 40-60fps on medium settings 720fps, the game drops to 15-20fps at random, bus usage jumps from 20% to over 65 and the GPU and CPU ms jump from 20 to 50-60. But the gpu usage stays at 95-100%. I am using...