It might not be the motherboard. You might want to replace the thermal paste in the device. Follow this guide, and then install new thermal paste onto the die of the heatsink [guide|158981]
Make sure not to strip any screws, as that could cause the heatsink to not have direct contact with the CPU. You will also want to be very carefull applying thermal paste, and make sure to cover the whole die, and not to scratch it.
Good luck fixing your device!