What was found
UPON CLIMBING THROUGH APPROXIMATELY 2500 FEET MSL IN A CLIMBING LEFT TURN DIRECT LAX VOR THE ENGINE NP CAST ILLUMINATED. IMMEDIATELY CALLED FOR THE QRH. MEANWHILE OBSERVED ENGINE NP IN EIS WINDOW RAPIDLY DECREASING ( INITIALLY 1650 THEN AS LOW AS 1300 RPM). DECLARED EMERGENCY AND REQUEST IMMEDIATE VECTOR TO RETURN TO HHR. WE WERE GIVEN A HEADING OF 090 AND DESCENT TO 2000 MSL. SLOWED THE AC TO 170 KIAS AND CALLED FOR GEAR EXTENSION FOLLOWED BY FLAPS 15, 30, AND THEN 40 DEGREES DUE TO OUT NEED TO LOOSE ADDITIONAL ALTITUDE WHILE ON HIGH FINAL. DURING THE DESCENT THE ENGINE NP CAS EXTINGUISHED AND ALL RPM/NP AND ENGINE INDICATIONS APPEARED NORMAL. LANDED WITHOUT INCIDENCE AND TAXIED BACK TO THE TERMINAL. NOTE, ALL ENGINE PARAMETERS AND PERFORMANCE WERE INDICATING NORMAL WITH THE EXCEPTION OF THE ENGINE NP AS DESCRIBED ABOVE.