Couple of years ago over Indian Ocean flew into a bubble of warm air, (just a whisper of cirrus clouds), A342 stopped flying and had to descent 4000' into clear air. About 15-20 min later temp recovered and were able to climb back to FL370.
On the other side, FL410 over the Alpes 18 months ago, light A346 (close to TOD), TAT decreased for 15 degrees and a barber pole just jumped down, leaving us with overspeed warning for 10-15 seconds, thrust on idle, just sitting there and enjoying the horn. Clear air.
In 15 yeras of long range flying happened to me twice. Although not common, sudden temperature variations are possible.
Closer to subject, third pilot was not present in the cockpit at that time for sure - somebody mentioned that he might have been sending ACARS massages. Third pilot (or a pilot) was in the bunk at that time, 4 hrs into the flight, and probably unable to reach the cockpit at all. Could have been captain himself, as far as we know.