Help 6300 Power supply problem

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I have a 6300 that is acting strangely lately. The symptoms are:
1) Machine powers up ok
2) after a random time interval (ranging from 5min to several hours)
   it just turns off. The fan is still working but everything else has
   no power.
3) Turning it off and waiting for another random time interval
   (ranging from seconds to 2hours) it can be turned on again
   with same symptoms.

I have the following questions:
1)does anybody have any idea about the cause of such behavior
2)does anybody know a place I can have it fixed within an hours
  drive from Princeton?
3)How easy is it to change the supply myself and where can I get
  a new supply (I prefer att/olivetti )

Thanks in advance
John Vlontzos
vlo at taipei.princeton.edu