Strange graphical performance issues

While running the game on medium presets I achieve 28-30 fps with minor stuttering on Yas Marina. While running on the high preset, I achieve 47-60 fps with moderate stuttering on Rio and major stuttering on Yas Marina, but every other track is perfectly fine. While switching to custom settings I am able to achieve a solid 60 fps, except with major stuttering on Rio and Yas Marina. I suspect that Yas Marina has issues due to a high anti-aliasing requirement, but I’m not sure why Rio is affected by this lag. Any ideas??
My PC specs:
Intel Core i3-4170
16GB Kingston HyperX DDR3
Asus GTX 960 Turbo OC 4GB

This game has some major memory leak going on. When I start the game it’s as smooth as silk but after an hour it lags and stutters like crazy even though the fps counter shows 70fps - 80fps+. Restarting the game fixes this for me. I would suggest disabling news though. That’s a resource hog. I have AA disabled and it doesn’t look like AA is disabled.

Yea, I think I notice the memory leaks too. That would explain some of the stuttering at least. However, when I disable AA, the game has really choppy edges, which I don’t like…