Problem w/ dimension 8300 not starting

I have been having problems with my Dimension 8300 (running XP purchased in 2004)starting.  When I hit the start button it will sometimes not do anything - acts as if it is not getting any power.  I discovered by accident that if I cut the power to it (by either unplugging it or shutting off the surge protector strip) for ten seconds or more and try it again it will than start up as normal.  Or at least this use to work - now it seems to require a longer and longer period unplugged to get it to start again (sometimes 8 plus HOURS).  Usually it will stay on fine for hours of use once I get it to start up, but sometimes it will just cut off (again appearing as if the power was cut) within a minute of it starting up. 

 

My guess is that it need a new power supply card, though someone suggest that it could be a faulty motherboard...  Any suggestions?

 

Thanks so much in advance,