Power Supply

My Aopen ATX 300W Power Supply works OK when first powered, but on reset, restart or turn off at PC switch then restart fails and there are 3 Beeps from BIOS. (MotherBoard is MSI KT333 #1, AMD Duron 1G Pcu, 256Mb DDR PC2100 Ram.) CPU fan is operating OK.
Disconnect Mains Power from the power supply, then re connect Mains power to the PSU and switch PC on and it Boots up correctly and operates correctly until switched off, reset or restarted.
Mains power MUST be completely cut from the PSU before a restart is possible on ALL occasions.
Warranty has been rejected Service Distributor claims PSU is OK. (Aopen Case and power supply purchased just 14 days ago today.)
Another Power supply fitted and all works as it should.