A B767-300 left Air Data Computer failed in flight causing the Captain's airspeed to decrease greatly while his altimeter slowly dropped. The First Officer recognized the error and after ADC ALTN was selected the flight continued in RVSM with a single altitude source.

Narrative

Approximately 7 hours into flight; noticed the throttles advancing significantly. Scanned instruments to see the Captain's altimeter was 300 FT low and the airspeed was decaying through 200 KTS. The aircraft pitched up a few degrees and began to climb. Captain disengaged autopilot and autothrottle and pushed the throttles to the stops. The airspeed continued to drop to 120 KTS. The First Officer claimed his airspeed was at barber pole. A quick cross scan verified high speed and 300 FT above altitude on his instruments. The aircraft was leveled off; the throttles reduced; and the aircraft was given to the First Officer to fly. Left ADC failure was suspected; the Captain's alternate ADC was selected and initially some of the instruments returned to normal. Dispatch and maintenance were contacted; and the appropriate checklists were performed. Based on a consensus; we elected to proceed on to our planned destination pending no further degradations. The purser was briefed for purposes of awareness. A cabin advisory was determined not necessary.