Have experienced the very same issue here. Select a long flight and engage auto-pilot, say if doing a flight from Orlando (USA) to OR Tambo in South Africa, flying Boeing 747-8. (About a 15hr flight, using default plane) Fps eventually drops after a few hours from 60Fps down to about 14Fps… GPU usage drops to around 18% +/-… I have checked all power saving options in Windows, and turned off whatever power saving options there are, but this does not help…
Windows 11 Pro - 21H2 - 22000.675
Intel i9-11900k
RTX3080 - nVidia Driver Version 30.0.15.1259
64Gb RAM
2Tb 980 Pro (Samsung)
Regards Mark