The laptop is a 4 years old Asus N56VZ, ambient temperature is around 25°C.
I've already unmounted the laptop, cleared the (not much) dust and changed the original thermal paste with Arctic MX-4, but the temperatures won't drop. I'm on Windows 10, I've also checked the temperatures with a clean installation of Ubuntu 14.04 LTS and they're pretty much the same. The situation seems to be pretty critical given that if I start a system scan in Bitdefender the computer will turn off automatically after ~20 minutes because of the activation of the thermal protection.
I specify that the CPU used to have normal temperatures, around 40-45°C in idle and 80°C in heavy load, before that this problem started. If it is not the OS/loads, the dust or the thermal paste causing these temperatures, what can it be?