Gatway profile 3 shutdown problems

Hello I have recently got a gateway profile 3 computer it origanally
came with win 98 but when I got it. It had win 2000 pro but anyway it
did not have a power cord with it so I got a new one from ebay its not
the origanal cord but had the same watts just genaric I think.So I
plugged it in and worked fine for about 3 mins then the coputer just
shut down so I waited a few mins and tried again just kept doing the
same thing somtimes 3 mins somtimes 1 hour so I Formated the drive and
put win xp on it still does the same thing but now it will shut down
even before it fully loads windows?
Could this be a motherboard problem or hard drive
Or the ac cord here is the one if you need to take a look.
http://cgi.ebay.com/ws/eBayISAPI.dll...m=140192237991
Thanks for your time Any help would be GREATLY appreciated.