Stuttering only when I start in 2D and then switch to VR

You are absolutely right, a faulty PSU doing that? No way!

https://forums.flightsimulator.com/t/fs2020-powers-off-my-pc-at-the-load-screen-after-a-game-crash/306847

https://forums.flightsimulator.com/t/computer-reboot-problem-finally-resolved/284113

Etc.