THE 55 HOUR STUDENT PILOT PREFLIGHTED THE AIRPLANE AND NOTED FUEL IN BOTH TANKS, THE RIGHT TANK CONTAINING '15 TO 16 GALLONS.' HE TOOK OFF TO PRACTICE DOWNWIND LEG, THE ENGINE 'COUGHED' AND LOST POWER. THE PILOT SAID HE SWITCHED THE FUEL SELECTOR FROM THE RIGHT TANK TO THE LEFT TANK. THE ENGINE REGAINED POWER MOMENTARILY, THEN QUIT AGAIN. THE PILOT DID NOT RECALL THE IMPACT, BUT WITNESSES SAID THE AIRPLANE NOSED INTO THE GROUND. POSTACCIDENT EXAMINATION DISCLOSED 'A MEASURABLE AMOUNT OF FUEL' IN BOTH TANKS, BUT NO FUEL IN THE CARBURETOR BOWL OR FUEL PUMP.