A B737-300 First Officer reported the loss of his air data computer; resulting in erroneous airspeed indications; as well as warning flags for other associated components. The aircraft deviated from its assigned altitude by 500 FT; but the air data computer returned to normal in the descent.

Narrative

I was the pilot flying. At cruise while I was looking at the overhead panel to change the cabin temperature; the B autopilot disengaged. This was followed by all flags on my airspeed indicator; the altitude OFF flag and the vertical speed indicator displaying. My airspeed was indicating over Vmo; and my altimeter was climbing. In the short time that we recognized that it was probably an air data computer failure and transferred control to the Captain; I had gently reduced power and allowed the aircraft to descend no more than 500 FT below cruise altitude. The unreliable airspeed memory items and check list were accomplished. ATC was informed of the altitude deviation; and we returned to our cruise altitude. I asked ATC if there was a loss of separation; and they said; 'No; and there is no one below you.' We asked for and then were cleared to 28;000 FT. As we descended; all of my instruments returned to normal. The aircraft did not over speed. It was only erroneous indications on my instruments. The air data computer was removed and replaced.