I've been getting these issues since S3 too.
Tried all pc drivers, bios, reinstalled game etc.
I'm running a couple of Deco Mesh M5 things, which if I remove, seems to sort my issue. With them in the network, I get 5-10 spikes a game, fps drops by 30 ish, TN spikes to the top of the chart for a second.
Take them out, played for hours without a single drop or spike.
I had them in since launch, so no idea why it would be an issue now. And clearly isn't going to be everyone's issue. I would imagine in the 'performance improvements' Dice changed the way something is handled, which is causing the TN issue.
5900X, 3080, 32GB. Capped at 141 for Gsync.