B777-200 CREW REPORTS MOMENTARY COURSE DEVIATION ON NORTH ATLANTIC RANDOM TRACK.

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AT THE TIME OF THE INCIDENT; THE CAPT WAS ON REST BREAK. I WAS ACTING AS RELIEF PLT IN THE L SEAT. ON A RANDOM NAT RTE OVER THE ATLANTIC; APPROX 58 NM W OF N57W040; THE ACFT BEGAN AN IMMEDIATE R TURN TOWARDS N; OFF THE TRACK WHILE FLYING IN LNAV. THE TURN WAS STOPPED AFTER APPROX 25 DEGS OF DEV BY SELECTING HDG MODE. A 50 DEG INTERCEPT COURSE BACK TO THE NAT WAS IMMEDIATELY INITIATED. WE THEN NOTICED OUR NEXT WAYPOINT ON THE RTE (N58W050) HAD DROPPED OUT OF THE DATABASE. WE REINSTALLED THE COORDINATES; VERIFIED THE COURSE TO THE COORDINATES AGAINST OUR FPF AND REJOINED THE TRACK IN LNAV. DURING THIS INCIDENT OUR MAGENTA COURSE LINE REMAINED ON SCREEN AS DID OUR 1 DEG OFFSET LINE TO THE R. NO FURTHER ANOMALIES WERE ENCOUNTERED AND WE BRIEFED THE CAPT OF THE INCIDENT WHEN HE RETURNED OFF BREAK. THERE WAS NOT A GROSS DEV DURING THE INCIDENT -- THE OTHER FO AND MYSELF BOTH AGREED THAT WE REMAINED WITHIN 6-10 NM OF THE ASSIGNED NAT. WINDS AT FL340 WERE PREVAILING FROM APPROX 180 DEGS AT +130 KTS AT THE TIME OF THE INCIDENT. MAINT CTL WAS NOTIFIED IMMEDIATELY AS WELL AS OUR DISPATCHER. AFTER LNDG; THE CAPT CONTACTED THE ON-DUTY FLT OPS SUPVR TO ADVISE HIM OF THE INCIDENT.