Hardware - like others have suggested...make sure your part is decoupled, at min of a .1uF cap.  Do you have the MCLR pulled up?  In all my years, its rare to come across a chip that you say is plain bad....there have been a few, but its either bad or good...if it runs once, it usually doesnt go evil...
   
  software.  As suggested, write a small ISR that blinks an LED so you know if the code is executing.  Do you have the WDT off?
   
  I'd lean to the issue being in the software side of things.

 
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