Hm strangely, but something similar happend to me too. I launched LFS and minimized it to tray, because i was doing something else at the moment. After a while i went back to lfs, full screen mode and it was like game was runing on very low fps and then smoothly again and so on. So i quit the session, reloaded the track and there it was again. Didn't happen to me when i just launched lfs and went racing immediately. And i didn't get this ever before. And no i didn't make any changes to my comp whatsoever. Happend to me only once tbh, but i didn't get this in previous versions.