my guess is either you have some seriously unreliable power in your location or perhaps the motherboard is electrically shorting against the case. it could also be entirely possible you have just been unlucky enough to get two dud power supplies, it does happen.
make sure the motherboard is not touching the case with any electrical contacts AND that the screws/posts that mount it (i hope you used these
) aren't contacting any electrical contacts either. if a third one blows and you're sure it wasn't shorting then i think we can rule out coincidence and shorting which would lead me to believe you just have poor mains power. a UPS (Uninterruptable Power Supply, also called power conditioners) may solve the problem.
as an after thought, are you certain the switch at the back of the power supply is set to the correct voltage for your locale?