Around 5 hours into cruising, I suddenly experience a loss of airspeed. It's just unrecoverable. Thrust level is maxed, but the airspeed just keeps dropping. Is anyone experiencing the same or know how to solve it? Please note that this doesn't happen on 2-3 hour flights.

What weather were you using. In the past if there was a problem with weather injection the ISA deviation would go to something insane like ISA +60 and the thrust would die.