I had the same thing today when flying from KMIA to EGKK.
The fps and GPU usage gradually decreased as time went on. GPU and CPU temps were all well within normal range at ~50C. The fps was around 5 and GPU usage peaked at 40% but dropped to 0% frequently at mid point of test. Frame time was between 90ms and 200ms. RAM usage increased from 6GB to 14GB and audio started crackling during test.
Interestingly when my monitor resumed from sleep during the test the aircraft and environment audio from MSFS failed to work and RAM dropped to 3GB before slowly increasing again despite no audio except menu clicks working in the sim.
Test was stopped before making landfall in the UK as fps was hovering between 0 and 5 fps. Frame time was between 150ms and 400ms at this point. Sim was very unstable and clearly not working well enough to even bother attempting a landing.
The dev mode fps counter was flipping between limited by CoherentGTDraw and CoherentGTUIThread.
I do have Toolbar Pushback installed. No pop out windows were open at time of checking fps.
Mods in use during test:
Salty 747-8i with BA livery
Toolbar Pushback
PC Specs:
CPU: AMD Ryzen 7 5800X
GPU: AMD RX6800 XT
RAM: 32GB
Related Zendesk Ticket: #158684