I have run into the same issue as well! This started well before SU5 and SU4. This always happens on ultra long haul (I’d say >10 hours) and it doesn’t matter if you are over polar regions or not. I can confirm that none of the suggestions has worked so far (ai traffic, live weather, lower graphics settings, no community content, etc.) and this has to be a bug in their programming. It also happens to me that whenever I pause, everything just goes back to normal.
The other day I was flying KMIA-VTBS and it got stuck over the north pole with the GPU utilization jumping between 0~20% and FPS between 1-10.
The only solution I have found so far is by breaking apart your flight plan into smaller chunks and feed them to the MCDU as you fly. This can sometimes prevent the game from getting stuck. I have done ZBAA-SAEZ twice successfully with the entire flight plan broken into segments, ZBAA-RJAA, RJAA-PHNL, PHNL-SAEZ. I suspect it has something to do with the programming of their nav system…it seems that the MCDU just cannot handle long flight plans.
Just my two cents.