AN MD88 AIRLINE CREW OVERSHOT THEIR ASSIGNED ALT DURING A DSCNT; IN PART; DUE TO A RUNAWAY CRUISE TRIM NOSE DOWN.

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WE WERE GIVEN A XING RESTR OF 20 MI S OF FLM VOR AT FL220. WE STARTED DSCNT AT 30 MI S OF FLM. THE CAPT WAS FLYING WITH THE ACFT ON AUTOPLT. DSNDING THROUGH FL230 FOR FL220 THE ACFT STARTED PITCHING DOWN MORE INSTEAD OF PITCHING UP. I WARNED THE CAPT TO BE PREPARED TO MANUALLY LEVEL OFF. THE CAPT TRIED TO LEVEL OFF SMOOTHLY BUT IN DOING SO THE ACFT MOMENTARILY DSNDED 400 FT LOW TO FL216. DURING THE LATER PHASE OF THE LEVELOFF WE NOTICED THAT THE SECONDARY TRIM WAS STUCK ENGAGED AND SLOWLY RUNNING AWAY IN THE NOSE DOWN POS. THIS GAVE THE CAPT GREAT DIFFICULTY IN THE LEVELOFF UNTIL THE PROB WAS RECOGNIZED AND WE SWITCHED TO PRIMARY (FASTER) TRIM. WE WROTE UP THE PROB FOR MAINT AND CONTINUED UNEVENTFULLY TO DAYTON. ZID DID NOT MENTION OR QUESTION THE MOMENTARY ALTDEV TO US. NO TFC CONFLICTS WERE NOTED VISUALLY OR ON TCASII. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE FO SAID THAT HE HAS ENOUGH EXPERIENCE IN THE MD88 TO KNOW THAT HE SHOULD BE PREPARED TO TAKE ACTION TO INSURE THAT ALTS ARE CAPTURED BY THE AUTOPLT. DURING THIS INCIDENT; THE FO THINKS THAT HE WAS LISTENING TO THE ATIS OR DOING SOMETHING ELSE WHEN HE NOTICED THAT THE DSCNT RATE SEEMED EXCESSIVE FOR THE AMOUNT OF ALT REMAINING BEFORE THEIR ASSIGNED LEVELOFF ALT. HE SUGGESTED THAT THE CAPT MANUALLY LEVEL OFF; BUT INSTEAD HE INITIALLY ATTEMPTED TO USE THE AUTOPLT PITCH CTL WHEEL. AT THIS TIME; THE FLC NOTICED THAT THE AUTOPLT TRIM WAS STILL MOVING IN THE NOSE DOWN DIRECTION. THE CAPT THEN DISCONNECTED THE AUTOPLT AND USED THE MAIN ELECTRIC TRIM AND CTL WHEEL DISPLACEMENT TO CLB BACK TO THEIR ASSIGNED ALT. THE CRUISE TRIM CIRCUIT WAS DISCONNECTED ACCORDING TO THE CHKLIST AND THE PROPER ENTRY WAS MADE IN THE MAINT LOG. THE FO SAID THAT HE WAS UNABLE TO FOLLOW UP ON THE MAINT ACTION BECAUSE THEY CHANGED ACFT AND HE HAS NOT FLOWN THAT ACFT SINCE.