A B737-400 flight crew established control of their pressurization system in the STBY mode after the AUTO mode failed to provide the appropriate rate of climb and/or descent scheduling.

Narrative

Climbing through FL180 the pressurization system attempted an unscheduled pressurization change. The cabin rate of climb should have been ~500 FPM. The system was instead indicating a rate of descent of 500 to 600 FPM; and the cabin differential pressure was above 8.2 PSI; well into the yellow arc. We decreased our rate of climb and ATC cleared us to level out at an intermediate altitude. Above FL230 the differential pressurization indicator was nearing the red line; and the cabin rate of climb was still fluctuating between 500 and 600 FPM descent. There were no warning lights on. After leveling off we coordinated a descent with ATC to relieve the differential pressure. The First Officer flew the jet while I accomplished the AUTO FAIL or Unscheduled Pressurization Change QRH checklist. After I placed the Pressurization Mode Selector to STBY the cabin altitude was controllable; and we resumed normal operations. After discussions with Maintenance and Dispatch we agreed to continue normal operations and proceed to the scheduled destination.Dispatch provided our flight with a contingency fuel burn plan should a lower altitude become necessary. We carefully monitored the pressurization operating in the STBY mode. After coordinating with ATC; we climbed to our flight planned altitude and continued to our destination. Throughout the remainder of the flight the pressurization system performed flawlessly. We wrote up the system and turned the aircraft over to Maintenance at our destination.