It's not just the CPU, the main cause it's the GPU, because this laptop has a integrated intel HD graphics 4000 if i'm not mistaken and the dedicated one radeon 7XXX HD depending on model. The thing is, when running on Windows 8 ,or Windows in general , it happens that in low - medium demanding processes, it runs on the integrated one, which is not weak at all, but it runs on it for generating less heat to the whole system. When running higher demanding processes such as top notch games, it switch to the more powerful one , the Radeon. The thing is that on Ubuntu, this doesn't happen. BOTH of the GPU's are running, overall generating more heat to the whole machine. This is the cause for overheating on ubuntu, as i'm not a master ubuntu user, i don't know exact coordonation for solving the problem, but this is what happens, i hope i cleared the situation for you a bit. Cheers !