Specific motherboards best?

I've been using an older Asrock board that has 2 x16 PCIe, 1 x4 and 2 x1.

I've got a 5700 in the x16 slot which is mining and serves as the display out, and then a vega56 via adapter to the other x16 slot, and a 5700xt via adapter from the x4 slot.

Anyway, the issue is, that if I add another card, so a 4th card, to the x1 slot via adapter, the thing either wont boot or is very unstable and wont mine, or doesnt recognize the card.

Even with my current setup with 3 cards the computer sometimes has to be reset a couple of times or it doesnt boot, aka, no POST.

In your guys' experience, is that possibly simply because of the age of the motherboard (BIOS is up to date)?Or could something else be going on?
Should I maybe look at a newer board, or something more specific to mining?
Would it be best to have a dedicated card/APU just for graphics?