what's your ambient temp?
if even the thermal paste is ok then maybe it's a faulty temp sensor? but on all devices...
what is the CPU/HDD/GPU load when it's idling?
what temperatures does everything start out at?
Perhaps your power supply gone broke and providing bad power to the components so they start generating extra heat or one of the components is responsible for all the heat in the case (maybe faulty motherboard)?
this seems really strange o_x