Odd gpu usage

THE GAME STARTS DROPPING FRAMES AFTER AN HOUR OF PLAY.

I'M PLAYING THROUGH THE CAMPAIGN AGAIN AND AFTER AN HOUR OF PLAY, THE GAME CLOCKS MY GPU AT FULL CAPACITY EVEN IF IM IN FREE ROAM WITH NO CARS ON ROAD.

I HAVE TO RESTART THE GAME THEN IT RUNS FINE

IT'S NOT A TEMP ISSUE AT MY CPU DOESN'T EXCEED 80C NOR DOES MY GPU. PLS HELP