What was found
AIRCRAFT WAS GROUNDED: PILOT REPORTS AT CRUISE WITH #1 A/P SELECTED WITH ALT CRZ IN ALTITUDE WINDOW AND SPEED SET IN MANAGED WITH .79 MACH SET IN PERF, AIRCRAFT SUDDENLY ACCELERATED ABOUT 2 KNOTS INTO THE BARBER POLE GENERATINGS OVERSPEED SELECTING LOWER MACH IN OPEN SPEED BROUGHT NO REPONSE W THRUST LEVER PULLED OUT OF CLIMB DETENT AND SPEED REDUCED TWO ATTEMPTS WERE MADE TO RESET A/C TO MANAGED SPD BUT TIME POWER WOULD ONLY INCREASE BACK UP FULL CLIMB POWER WHEN THRUST LEVER POSITIONED TO CL DETENT SELECTED A/P #2 AND TURNED A/T BACK ON AIRCRAFT RESPONDED CORRECTLY IN BOTH MANAGED AND OPEN SPEED. MAINTENANCE REPORTS POST FLIGHT REPORT-NO FAULTS. DFDR INDICATED FCU FAILURE. REMOVED AND REPLACED FCU IAW AMM 22-81-12 AFS & LAND CAT3 CAPABILITY TESTS GOOD. NO FAULTS.