Hi guys. I am going to be honest, I am not a FS guy. I found this post by googling the exact same problem I am having with my 6900 XT in other games. It will get great FPS then lag for a split second out of no where. I have tried everything. I do not think I am getting bottle necked at all. I have a 5900x, 32GB 3600mhz ram, a gen4 nvme ssd and a 850w Gold PSU.
Any solution? Seems to be a common problem.
I had similar issues. I think it was down to the immature usb and pcie4 drivers. I had to upgrade the bios on the motherboard, upgrade the usb drivers too, then enable pcie4 and the variable bar stuff in the bios. Only now in the last month have I had a system that runs stably in pcie4 config.
Am running an Msi x570 tomahawk, ryzen 5600x, 32Gb 3600 MHz RAM, RX6900xt, Corsair mp600 pcie4 drives.
Had to run it in pcie3 mode till msi updated the bios on the Moro to turn in pcie4 and for it to run without the temp freezes followed by really low FPS.
Before any words of wisdom on the match of components. It was 2020 I bought what I could get.
For AMD systems see my post on hyperthreading re disabling some cores ā¦ I am now stutter free
Always fun reading these threads a while after the conversation has played out, and witnessing the inevitable ācollision with terrainā in slow motionā¦
Thanks for your useful, analytical and experience-based assessment of 6000 series performance, Maxisone - I found it helpful 
5900X (PBO +200mhz) + Aorus Ultra x570 (rev 1.2) + Radeon 6900X (AMD edition) + 32 GB CL14.
Upgraded my 5 years old system without reinstalling windows! Finished today.
Except few crashes no problems for me. Everything is running default. Temps are generally between 50-60 degrees Celsius. All settings ultra (also 8x8 and 2048 terrain Shadows for testing).
Flight Simulator cannot utilize hardware yet, waiting for july 27 update. Neither GPU or CPU goes over high % usage for long. (Resolution 2k).
My honest opinion this is so stupid. My previous setup was 6700k with 1080. I have been playing since Tech Alpha and has less than 5 crashes so far. With this upgraded system 3 crashes so far. I hope DX12 and engine rewrite solves this weird problems.
EDIT: Forgot to mention, i defined custom fan curve because with default one it was around 75-80 degrees even tho i have perfect case airflow. Now with custom curve, never see 70 degrees and limited fps, disabled all enhancements from Radeon UI, looks like stable so far.