3:24:53. At this point, watch his flight track. Trouble. This looks like a loss of engine(s).
What time, precisely, was his Mayday?
What jumps to mind is a pair of piston engines at eleven thousand feet for two hours, temp stabilized, being “cooled” by relatively thin air. What follows is a descent into lower level, where air is thicker, cools better, and is in a marine environment.
Shock cooling. The bane of all piston pilots. These engines need close monitoring in descent. At low power, rapidly cooling, things can break. It wouldn’t be the first time an engine gave up completely. And losing both has happened.