B737-400 flight crew experiences cabin altitude warning at FL340 in an aircraft that has a history of previous problems. Crew descends to FL200 where pressurization can be maintained with the out flow valve closed; and elects to return to departure airport.

Narrative

Depressurization at 34;000 FT. Noted previous write up associated with same issue in prior days. Climb uneventful. Pressurization checked going through 10;000 FT. Pressurization checked at level off at 34;000 FT. Cabin altitude showing approximately 8;000. 5 minutes later approximately intermittent warning horn sounded. Memory items; QRH accomplished. Standby pressurization selected; noted outflow valve closed. Emergency declared with decent initiated towards 28;000 FT. Cabin altitude noted at FL280 was 11;000 FT. Continued decent towards FL240. Cabin altitude noted was 10;500 FT. Continued decent towards FL200. Cabin altitude noted was now acceptable at 8;500 FT. Outflow valve still noted closed as was during the entire decent. Stopped decent at 20;000 FT; reviewed checklists run; checked fuel and weather and contacted Dispatch. Dispatch advised a return to ZZZ. Rest of flight at FL200 until decent to ZZZ uneventful with operation dealing with pressurization normal at FL200. Canceled the emergency at FL200 when pressure stabilized.