Are all the wall sockets in your house measured at around 247v constantly? If yes, my guess is your house probably situated near a TNB substation. Although 247v is on the high side, there's no need to be alarmed as all you might experience are fans runs slightly faster, water heater runs slightly hotter etc. What you really need to take notice is if the measurement at the wall power outlets are on the low side (below 220v). Below are examples that causes low supplied voltage :

1. Wiring too old & degraded.
2. Termination or point of contacts oxidation.
3. Using power extension with low quality copper wiring or steel wiring (rare but exist)
4. Sharing the same power outlet with a high power appliance.

Constant low supplied voltage causes electrical appliances to fail prematurely due to overworked & overheated, wiring to catches fire etc.

Any corrections or additional info from experts are welcome.

Are all the motherboards that failed came from the same computer or power supply? Normally if supplied voltage is causing problems, the 1st thing that should be affected is the power supplies & not the motherboard. Is there any other electrical appliances that failed frequently?
Anyway if you suspect the culprit is the supplied voltage, an AVR from a good brand/manufacturer will ensure your electrical appliances gets the voltage within spec.