Clevo Laptop keeps shutting down

Clevo D40EV keeps shutting down and switching off, initially after booting into Linux environment or whilst loading Windows. Then won't even get to the launch screens of either. This seems to get better after leaving it but then quickly returns.

The fans come on intermittently even when changed to constant in the BIOS. When checked the ''radiator'' is hot once its switched off. I have cleaned and replaced the thermal compound with Arctic V silver and this has made little or no difference.

I am thinking it is a problem with the motherboard or just down to poor ventilation (although all the fans have been thoroughly cleaned!) Is there someway of testing the motherboard as it won't boot up, or is it possible and cost effective to replace if it is faulty?