A B737-NG CREW RPTS AN UNCOMMANDED AUTOPILOT PITCH UP ON APCH AFTER GEAR DOWN SELECTION WITH APCH MODE ENGAGED.

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A COMPANY PROCS APCH WAS BRIEFED DUE TO A RAIN SHOWER OVER THE AIRFIELD. WE COULD STILL SEE THE RWY AND AIRFIELD BUT KNEW VISIBILITY COULD GET WORSE. I (CAPT) WAS FLYING AND USING MY AUTOPLT. ATC HAD GIVEN US A VECTOR THAT WOULD INTERCEPT THE FINAL APCH COURSE ABOUT 2 MI OUTSIDE THE FAF. WE WERE ON AN ATC ASSIGNED 300 DEG INTERCEPT HDG TO THE RWY 27 LOC COURSE. I CALLED FOR LNDG GEAR DOWN AND EITHER HAD CALLED FOR OR WAS ABOUT TO CALL FOR FLAPS 15 DEGS. I SELECTED 'VOR/LOC' ON MCP AND ALMOST IMMEDIATELY CHANGED MY MIND AND SELECTED APCH MODE SINCE WE WERE GOING TO INTERCEPT THE COURSE PRIOR TO GS. RIGHT AFTER SELECTING APCH MODE THE NOSE OF THE ACFT BEGAN A MODERATE SMOOTH PITCH UP AND AIRSPD BEGAN TO DECREASE. ALT STARTED TO INCREASE ABOVE THE MCP SELECTED 3000 FT WITH THE AUTOPLT STILL ENGAGED. I ADDED PWR AND PUSHED FORWARD ON THE YOKE (INSTINCTIVELY) TO STOP THE UNEXPECTED ALTDEV AND COUNTER THE AIRSPD LOSS. THE AUTOPLT RESISTED MY ATTEMPT TO PUSH FORWARD. AT THIS TIME I DISCONNECTED THE AUTOPLT AND ADDED MORE PWR SINCE THE NOSE WAS PRETTY HIGH AND WE WERE NOW WELL BELOW SELECTED FLAP MASI SPD. I ROLLED THE ACFT (L?) TO GET THE NOSE DOWN AND ONCE RECOVERED ASKED FOR FLAPS 15 DEGS. I BELIEVE THE ACFT MAY HAVE ALREADY STARTED THE L ROLL TO INTERCEPT THE COURSE. I ALSO REMEMBER HAVING TO TRIM NOSE DOWN. I GLANCED AT THE ALTIMETER AND IT SHOWED 3600 FT (600 FT ABOVE MCP SELECTED ALT). THERE WAS NEVER A STICK SHAKER OR STALL BUFFET. I CORRECTED BACK TO 3000 FT AND CONTINUED THE APCH MANUALLY. REMAINDER OF THE FLT WAS UNEVENTFUL. MAINT OR FDAP DATA MIGHT FIND A REASON FOR THE AUTOPLT PITCH UP AND PROVIDE A HARDWARE/SOFTWARE/PLT PROC FIX. PLTS SHOULD KNOW THAT THIS SITUATION COULD HAPPEN AND BE PREPARED BY READING ABOUT THE INCIDENT OR PRACTICING IT IN THE SIMULATOR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT RECEIVED A REPORT FROM THE ACR MAINTENANCE DEPARTMENT STATING THAT A HORIZONTAL STABILIZER TRIM SYSTEM POSITIONING SENSOR REPORTED A FAULT. THAT FAULT CAUSED THE AUTOPILOT SYSTEM TO SENSE A NOSE UP REQUIREMENT IN ORDER TO MEET THE CURRENT FLT CONFIGURATION.