A B737-300 DSNDED OUT OF RVSM AIRSPACE FOLLOWING AN AUTOPLT TRIM MALFUNCTION BECAUSE IT NO LONGER MET THE RVSM AIRSPACE REQUIREMENTS.

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CRUISING AT 33000 FT; THE STAB OUT OF TRIM LIGHT CAME ON. WE RAN THE STAB OUT OF TRIM CHECKLIST IN THE QRH; AND DETERMINED THAT THE AUTOPLT WAS NOT TRIMMING THE STABILIZER PROPERLY. WE NOTIFIED ATC; LEFT RVSM AIRSPACE. WE THEN NOTIFIED DISPATCH. WE CONTINUED ON WITH THE AUTOPLT DISENGAGED.SUPPLEMENTAL INFO FROM ACN 722005: WE WERE IN CRUISE FLT AT FL330; WHEN THE STAB OUT OF TRIM LIGHT ILLUMINATED AND THE ACFT SLOWLY BEGAN TO CLB. WE NOTICED THE MALFUNCTION IMMEDIATELY; AND TOOK OVER HAND FLYING THE ACFT. WE COMPLETED THE QRH PROC AND DETERMINED THAT THE AUTOPLT WAS NOT TRIMMING PROPERLY. WE NOTIFIED ATC OF THE EQUIPMENT FAILURE; AND THAT WE NEEDED TO LEAVE RVSM AIRSPACE.CALLBACK CONVERSATION WITH RPTR ACN 722002 REVEALED THE FOLLOWING INFO: RPTR STATED THAT BECAUSE THE AUTOPLT WAS REQUIRED TO BE DISCONNECTED AS A RESULT OF THAT ACFT'S PARTICULAR AUTOPLT TRIM FAILURE; THE ACFT COULD NO LONGER MAINTAIN ALT. AT THIS POINT THE RVSM REQUIREMENT WAS NO LONGER MET. THE RPTR ALSO STATED THAT ONCE THE ACFT WAS AT A LOWER ALT AND BEING HAND FLOWN; THE CREW BECAME FATIGUED BY THE REQUIREMENT TO MANUALLY MAINTAIN ALT AND THEY 'HAD THEIR HANDS FULL.'