Hi again,
Computer power supplies have various ratings for their MTBF. Some are 100,000 hours for example. You'd be better off
finding one that advertises their MTBF rather than picking one at random.
What can happen is the output caps can change drastically in value as well as their ESR goes up, and that can cause an
over voltage condition and the power supply will shut down. I dont think any power supplies are made today that dont
have an over-voltage circuit even if not advertised as such, but again it's good to find one that specifically states that
it does have this feature too.
Oh yeah, about the 12v (or other) possibly having a larger peak...
You can simply measure the output of the wall wart and see if it measures 12vdc or higher. You can measure
the ac voltage too to see if there is much ripple, although with some meters you'll have to put a cap like 0.1uf
in series with the meter to read this properly.