AN ACR B737-300 FLC HAS THE AUTOPLT CHANGE MODES DURING A LEVELOFF FROM A DSCNT AND SMOOTHLY PITCH DOWN 10 DEGS.

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ON APCH TO EWR ARPT; WE HAD BEEN CLRED BY ATC TO DSND TO 5000 FT FROM 6000 FT. ACFT WAS A B737-300; SPD WAS 190 KTS IAS; FLAPS WERE 1 DEG. AT APPROX 5300 FT THE FLT MGMNT ANNUNCIATOR WENT FROM PITCH EQUALLING VERT SPD; AUTOTHROTTLES EQUALLING SPD; TO ALT HOLD AND VERT SPD ARM IN PITCH. AT THE SAME TIME THE ACFT PITCHED NOSE DOWN 10 DEGS. I TOLD THE FO TO STOP THE DSCNT AND RETURN TO 5000 FT. HE DID SO IN A TIMELY MANNER; HOWEVER; WE REACHED 4800 FT BEFORE THE DSCNT WAS ARRESTED. THE EXCESSIVE DSCNT RATE CAUSED AN RA ON THE TCASII FOR STOP DSCNT. AFTER ENTERING VMC CONDITIONS AT 4000 FT MSL WE REENGAGED THE AUTOPLT. NO ABNORMALS WERE OBSERVED. ON LNDG IN EWR I WROTE THE PROB IN THE MAINT LOG. THE MECHS DID A BITE CHK ON THE AUTOPLT AND FOUND NOTHING ABNORMAL. WE FLEW THE SAME ACFT FOR 4 MORE FLTS AND HAD NO PROBS WITH THE AUTOPLT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THAT THE MAINT PERSONNEL AT EWR WENT OVER THE AUTOPLT AND THE RADIOS VERY CAREFULLY AND COULD NOT FIND THE CAUSE OF THE PITCH DOWN DURING THE LEVEL OFF PHASE OF FLT. HE DID SAY THAT THE ILS FREQ HAD JUST BEEN TUNED AND IDENTED ON HIS NAV RADIO; BUT HE WAS UNSURE IF THE TUNING MODE WAS STILL IN MANUAL OR AUTO AND HE WAS SURE THAT THE ILS MODE WAS NOT SELECTED ON THE AUTOPLT. HE AGAIN STATED THAT THE ACFT PERFORMED PERFECTLY THROUGHOUT THE REST OF THAT DAY'S FLTS. THE FLT ATTENDANTS DID NOT MENTION THAT ANYONE IN THE CABIN WAS USING AN ELECTRONIC DEVICE SUCH AS A COMPUTER; PHONE OR GAME.