Strange issue with ASRock B450M Pro 4

Hello,

I recently decided to put together a spare computer using old parts, including the above ASRock AM4 board.

The issue is it doesn't work properly with two different Zen3 CPUs I've tried, even though it's supposed to, with BIOS updates.

I updated to the newest BIOS and tried it with a 5800x, and the mobo exhibited several weird problems.

First of all PCIe has to be capped at 8x for it to display anything. I had to use the second PCIe slot to do this.

If you leave it at the full 16x, the GPU doesn't display anything.

Second, the PC restarts when waking up from sleep mode.

I assumed it had to be faulty CPU, so I sent it back for a refund and bought another one from another retailer. The exact same thing happened, so it's definitely the mobo.

The weird thing is it works perfectly with the old R5 2600 that was originally in the board. No problem at all, PCIe fullspeed, and no restarts.

Any ideas why this is happening and if it's fixable?

Cheers.