What was found
AT L/O FL 350 CI 60 COMMANDED MACH SLOWLY REDUCED TO .73 INTERVENED WITH SPD INTV AND SET MACH .79. ON BOTH FMC'S A -MESSAGE APPEARED STATING MAX MACH WAS .745. FMC COMMANDED MACH CONTINUOUSLY CHANGED, SOMETIMES APPEARING TO CORRECT TO NORMAL. ON CRZ PAGE OPT/MAX WERE CHANGING AND LOWERED & BOTH READING FL 345. ALL RETURNED TO NORMAL AFTER ABOUT 20 MINUTES. ACFT GROUNDED. ACCOMPLISHED FMC BITE CHECK IAW FIM 34-61 TASK 801 & ADIRS BITE CHECK IAW FIM 34-21 TASK 801. NO FAULTS FOUND. NO HISTORY PAST 30 DAYS OK FOR SERVICE.