A B737-300 Air Data Computer malfunctioned causing INOP FLAGS in the First Officer's airspeed indicator; altimeter and IVSI. The indications returned to normal eventually but the ADC was replace at the next station.

Narrative

The First Officer was the PF and the B autopilot disengaged. The First Officer took control of the aircraft and was trying to maintain altitude and airspeed; but he actually was descending. I called; 'Monitor your altitude;' because he was descending. At that moment; all the INOP FLAGS from his airspeed indicator; altimeter and IVSI were in view. I assumed control and started a smooth climb to FL360 after losing 450'. We notified ATC of the altitude deviation; he acknowledged; and informed us not to worry because there was no traffic around us. We requested to descend to FL280 to stay out RVSM airspace. The indication on the First Officer's panel showed a continuous climb and a speed increase. The descent was IMC conditions and when we passed FL330 on VMC; the First Officer's airspeed indicator showed 5 to 10 knots over the barber pole. The overspeed clacker sounded and then all his instruments returned to normal (the actual airspeed was M.74). We continued our flight and notified Maintenance Control and Dispatch through the ACARS. After arriving; Maintenance replaced the number two air data computer; and after that we continued our trip.