THE AIRPLANE TRANSPORT PILOT REPORTED THAT WHILE IN CRUISE FLIGHT HE EXPERIENCED A PARTIAL LOSS OF ENGINE POWER TWICE WITHIN A PERIOD OF ABOUT THREE MINUTES. HE WAS ABLE TO REGAIN ENGINE POWER BOTH TIMES, BUT THE SECOND TIME THE AIRPLANE IMPACTED ON A SNOW-COVERED MOUNTAIN SIDE BEFORE THE DESCENT COULD BE STOPPED. A TEAR DOWN AND ANALYSIS OF THE ENGINE FUEL PUMP REVEALED AN EIGHTH INCH LONG THREAD-LIKE METAL SLIVER LODGED UNDER THE LOW PRESSURE CHECK VALVE PLUNGER.