What was found
IN FLIGHT: BOTH FMC'S MOMENTARILY BLANKED OUT AND RESET. "WHITE A/T LIM" APPEARED ON THE UPPER ENGINE DISPLAY. AMBER "LND ALT" APPEARED ON LOWER RIGHT CORNER OF PFD NEXT TO ALTITUDE TAPE. WAYPOINT IN UPPER RIGHT CORNER OF ND SHOWED DASHED LINES THEN BLANK BEFORE RE-APPEARING. BOTH CAME BACK ON IN FLIGHT. GROUNDED - YES. ACCOMPLISHED FMCS OPS TEST REF B737-800 AMM 34-61-00-710-801. OPS CK GOOD, NO FAULTS FOUND. ACCOMPLISHED FMCS BITE REF B737-800 FIM 34-61 TASK 801 NO CURRENT FAULTS PRESENT. ACCOMPLISHED DFCS LRU REPLACEMENT TEST FOR FMC REF FIM 22-11 TASK 802 ALL TESTS PASSED. ETOPS GROUND VERIFICATION SUCCESSFUL.