power problems

I have just installed a kx600 motherboard with an athlon 1ghz cpu and have
encountered a problem I hope you can help me resolve.
When I first started the system after installing the new motherboard
everything started okay but when I cut the power with the power switch the
system would not start again. I had a look inside while I was pressing
the power switch in and noticed the fans were turning a little bit . I then
tried pulling the power cables from the CD player and hard drive and floppy.
Then after trying the power switch again the system started. There
is no set pattern to what power cables I pull out to allow the system to
start - I just keep on pulling cables out and eventually the system starts.
Once it is running the system will stay running and restart with the restart
switch but as soon as I kill the power it won't start again. A couple of
times after the system has been running I have inserted a CD and the power
has shut down instantaneously. I have tried swapping the restart switch and
the power switch around but still have the same problem . My power supply is
a MEC model ATX 350/2F and is only a couple of weeks old.

Any advice you can give would be greatly appreciated.

Regards
gregork