On a very fresh install of Ubuntu 14.04, it sometimes happens that when I close or minimize a window, Unity freezes. I can not interact with the panel or any application, only move the mouse around. A short time later, the notebook fans get very noisy.
When I change to a TTY and check what keeps the system busy, it's usually ibus and ibus-ui-gtk3 that use >100% CPU. RAM usage is normal. But I am not sure if that is related to my problem, because after killing ibus Unity is still frozen. Only killing Xorg helps.
I just use the system normally, web browsing, texting, programming. I have only installed few programs.
I installed all updates. I tried both, the open source and the proprietary driver for my AMD GPU (I have a Intel/AMD dual GPU). This only happens with Unity.
I've found a similar problem, but disabling ibus in settings and killing it does not solve my problem. I have used Ubuntu on this machine before and it did not behave like this.