What was found
ON A RNAV APP ANGEL 4 LNAV VNAV ENGAGED AT APPROX 13500 THE AIRCRAFT BEGAN A RAPID DESCENT NOSING OVER AND ACCELERATING TO 315. THE ALTITUDE ON THE APPROACH HAD A 12000 OR ABOVE THAT WOULD HAVE BEEN BROKEN UNLESS THE AP WASN'T DISENGAGED. GROUNDED - YES. PERFORMED FMC BITE TEST PER B737-900 FIM 34-61 TASK 801. NO FAULTS FOUND, GROUND VERIFICATION COMPLETE. NO HISTORY FOR 30 DAYS.