A B737-200 IN CRUISE AT FL310 DECLARED AN EMER AND DIVERTED DUE TO UNCOMMANDED 600 FPM DSCNT AND RESTR ELEVATOR FEEL CAUSED BY FAULTY ELEVATOR FEEL COMPUTER.

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AT FL310 WITH AUTOPLT 'A' ENGAGED; AIRPLANE BEGAN AN UNCOMMANDED 600 FPM DSCNT AND DEV FROM FL310. WE DISENGAGED THE AUTOPLT TO RETURN TO LEVEL FLT. ELEVATOR FEEL WAS RESTR/REDUCED DURING INPUT TO ELEVATOR CTL. REQUESTED A DSCNT TO ASCERTAIN HOW MUCH RESTR INPUT THERE WAS WITH REGARD TO THE ELEVATOR. DURING DSCNT; FIRST TO FL290 AND THEN FL240; WE MOMENTARILY EXPERIENCED A RESTR ELEVATOR. IT FELT LIKE AUTOPLT WAS STILL ENGAGED OR THAT IT WAS STUCK IN CTL WHEEL STEERING OR MANUAL REVERSION. GAVE CTL OF ACFT TO FO. ACFT RATE OF DSCNT WAS UNDER CTL AT ALL AIRSPDS; BUT INPUT TO ELEVATOR WAS MOMENTARILY RESTR AT TIMES. PLANNED DIVERSION TO BWI AND DECLARED EMER. RAN QRH FOR RESTR ELEVATOR CTL. DURING DSCNT OUT OF 15000 FT; FO SAID ELEVATOR FELT NORMAL; SLOWED AIRPLANE AND CONFIGURED EARLY ON APCH. DURING THIS EVENT; ELECTRIC TRIM OPERATED NORMALLY. THERE WERE NO WARNING LIGHTS ILLUMINATED FOR 'FEEL DIFFERENTIAL;' 'FLT CTLS;' 'AUTOPLT DISENGAGE;' OR 'STABILIZER OUT OF TRIM.' CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AUTOPLT WAS ENGAGED WHEN THE AIRPLANE STARTED THE 600 FPM DSCNT. THE RPTR SAID WITH THE AUTOPLT DISCONNECTED THE ELEVATOR FEEL WAS THEN DISCOVERED TO BE RESTR WITH REDUCED AUTH. THE RPTR STATED THAT 5 LBS OF FORCE WOULD BE NORMAL; BUT IT REQUIRED ABOUT 30 LBS OF FORCE. AFTER LANDING; THE RPTR SAID THAT MAINT SUSPECTED ELEVATOR CTL CABLES WERE FROZEN OR RESTRICTED BY ICE DUE TO A POSSIBLE LEAK IN THE AFT LAVATORY. THE RPTR SAID THE ELEVATOR FEEL COMPUTER WAS TESTED; FOUND MALFUNCTIONING; AND WAS REPLACED.