|Yeah it's not burning out when it's off; it must be happening when you shut it off or when you turn it back on. Is it connected through a UPS or surge protector?|
If the problem is not at the AC input then it must be inside the computer case. Something there must be trying to pull too much current from the PSU. Maybe a short on the motherboard.
If they're covered under warranty and it's practical to send the PSU's back, I'd go ahead and do that, although you may want to keep one and open it up and see if the PSU internal fuse is blown.