But PSU is the way to bet.
I've replaced about half a dozen failed power supplies for friends in the last 12 months, and not met a single motherboard failure.
I keep a couple of spares in the cupboard, ready!
If you're replacing it, get the highest-rated one you can fit in the case. I buy 500 watt ones now - not a lot dearer than 300 watts, and a lot less likely to fail through overload.