What was found
DEPARTED, APPROX 30 MINUTES INTO FLIGHT. LT NAV DISPLAY VTK BOTH FMC CDU WITH ACARS PROMPT. LOSS OF NAV DATA ON L NAV DISPLAY. RT NAV DISPLAY LT NAV INFORMATION REMIAINED. AUTOPILOT TO CWS/CWP. 20-30 SECONDS LATER LT FMC RETURNED TO NORMAL WITHOUT CREW INTERVENTION. ON POS SHIFT PAGE 3. FMC LT AND RT 0.0 NM, GPS LT AND R 0.0 NM, IRS LT 1.8 NM, IRS RT 2.2 NM. ANP REMAINED 0.3 NM. NO OTHER ANOMALIES NOTED DURING REMAINDER OF FLIGHT. ACFT GROUNDED. ACCOMPLISHED FIM 34-61 TASK 836. NO DISCREPANCIES NOTED. WHILE CYCLING LT FMC CIRCUIT BREAKER, THE C/B SEEMED TO BE WEAK WHEN LATCHING CLOSED. REPLACED LT FMC AND CDU CIRCUIT BREAKERS. SYS OPS NORMAL.