Rendering suddenly became really slow at some moments

I'm recording gameplay footage in Nvidia Shadowplay and have been doing it for a year without problems. Recently I found out my renders wouldn't finish in 9 hours, it just get's stuck at some point and starts rendering 1 frame in like 10 minutes, really slow. Before that it took 2 hours to render he whole video.

And the funny thing that it can render like usual, I can see it go through the video at normal speed in Media Encoder, but then gets stuck at random moments. Sometimes in a few hours it can get going again at normal speed.

However, there are a few most problematic spots. My videos start with fullcam intro from my camera, it renders ok, but than when it gets to the first frame of gameplay + webcam - render get's stuck. Sometimes when I have multiple recordings of gameplay - it get's stuck on the cuts between them.

I have RTX 4080, i9 13900K, 32gb RAM.

And the funniest thing is that my other PC renders videos much easier, though it has only RTX 2080. It still gets these problems sometimes, but not that often. And this thing appeared suddenly, nothing has changed in my setup.

I tried clearing media cache, changing the codecs, rendering in small chunks - still have the problem. And the graphics card usage drops to 0-2% during that stuck moments.