What was found
AT CRUISE ALT, FL 340, WITH L NAV & V NAV ENGAGED THE ACFT DROPPED INTO "CWSR" AND ALT HOLD. BOTH FMC DISPLAYS WENT INTO "MENU" DISPLAYS. THERE WERE NO OTHER FMC/NAV CAUTIONS OR LIGHTS PRESENT. SYSTEM WAS RESTORED TO NORMAL OPS. ACFT GROUNDED. ACCOMPLISHED DFCS BITE IAW FIM 22-11, TASK 801, FLT LEG 1 ON A SIDE LOGGED 3 EA 22-11071 AND 22-11822, B SIDE, NO FAULTS. ACCOMPLISHED FIM 22-11, TASK 811 FMC PROBLEM CURRENT STATUS PASSED. ACCOMPLISHED FMC BITE PROCEDURE REF FIM 34-61, TASK 801 BOTH FMC'S PASSED, NO FAULTS LOGGED. ALL GROUND TESTS PASSED.