Umm, it does indeed seem like a system performance issue. Most playback problems, but not all I might add are caused by slow media/system drives, and or slow systems.
My system is quite powerful, but from time to time my system goes on strike, causing playback to be stuttery or just downright eratic!!
I’m not aware of any 720p 60fps encoding problems, but I will look into it.
All the best
Dave