suddenly restart our computers

When we're using computers. suddenly restart our computers so often what we do,
first
Check whether the power supply has a power that remains stable or is there a decline. how can replace the power supply while the others if it's in our home or we can borrow dulu deh ....
second
It's when you notice its existing components on your motherboard, is there a component (its usually called the "condenser" forms such as batteries of small tubes) which have swollen because flow is unstable.
third,
Clean the motherboard of dirt perhaps can still help ...

thanks ....