DURING FLT AT 11,000 FT, THE PLT NOTED THAT THE FUEL FLOW GAGE WAS 'WIGGLING' & THE ENG 'DIDN'T SOUND REAL HEALTHY.' HE INCREASED THE MIXTURE SLIGHTLY & TURNED THE AUX FUEL PUMP TO 'LOW BOOST.' THE FUEL FLOW THEN STABILIZED 'PRETTY GOOD.' THE PLT SAID THAT A LITTLE LATER, THE FUEL FLOW GAGE'WAS STILL FLUCTUATING AROUND' & THE ENG DIDN'T SOUND RIGHT, SO HE SWITCHED THE AUX FUEL PUMP TO 'HI BOOST' & LEANED THE MIXTURE SLIGHTLY. THE ENG THEN RAN NORMALLY & HE CONTINUED TO THE DESTINATION & LEFT THE AUX FUEL PUMP AT 'HI BOOST.' ON FINAL APCH TO LAND WITH THE AUX PUMP STILL ON, THE ENG LOST POWER. UNABLE TO CLEAR OBSTRUCTIONS, THE PLT ELECTED TO LAND IN A FIELD SHORT OF THE RWY. DURING ROLL-OUT IN TALL GRASS, THE ACFT STRUCK A MOUND OF DIRT & NOSED OVER. NO PREIMPACT FAILURE WAS FOUND & FUEL WAS STILL REMAINING. ACCORDING TO THE OWNER'S MANUAL, FUEL VAPORIZATION CAN OCCUR; IF FUEL FLOW FLUCTUATES ABOVE 1 GAL/HR, POSITION AUX PUMP TO 'HI' & ADJUST THE MISTURE; WHEN FUEL FLOW IS STEADY FOR SVRL MIN, AUX PUMP CAN BE TURNED OFF & MIXTURE RESET.