Multiple Power supply failures

I have a Sony RB 38G computer that has had 3 power supply failures in
the last four months. The first one was 2 months after warranty
expired. When I had it replaced, the guy said that a connector on the
motherboard was a little burnt. He said he cleaned it up and that
there should not be a problem. He replaced it with a 450 watt Okia.
Three weeks later it went bad. He replaced it under the 30 day
warranty. Two months later it has failed again. Should I replace the
power supply with a better one than Okia, or should I replace the
motherboard and power supply? If I replace the motherboard, are there
any suggestions as to specific motherboards for this computer?