Gigabyte Aorus X570 I Pro Wifi + Sapphire Radeon RX 5700XT running PCI-E 4.0

[yumanoid]

Does anyone here have this combo running at PCI-E 4.0 speeds? I have a Xtia Xproto case which came with a PCI-E Gen3 riser. I bought a PCI-E Gen4 riser off AliExpress (ADT-Link). I'm now trying to make it work, but it's not having it. The BIOS, while having the PCI-E set to either Auto or Gen4, will only start as Gen4 every n-th time, and when it does, everything has a horrible lag/stutter, even in BIOS. It doesn't crash/hang, it can boot windows, but everything is 10x slower and stutters. All the other times it will just start as Gen3 and with that everything is fine.
I used to have a different board before, the same chipset/form factor from ASRock. I seem to remember that board was actually able to boot to Gen4 (the same GPU), though I can't remember if it was with the current case or the previous one (Dan case) and therefore with a different riser.

 

Anyone has any ideas what this is? 

- the riser is shit?

- all the PCI-E Gen4 devices don't play nice together (GPU + two NVMe SSDs, both PCI-E Gen4)?