Question Problem With GPU (I Think)... Display Only Turns on Occasionally

[MarsColonist]

I've been having this problem with a new PC I put together, and I think the cause is the GPU, for reasons I lay out here:
Question - Problem: Display Comes On, Then Goes Out | Tom's Hardware Forum

Fortunately, I happen to have an old AMD GPU in an old PC laying around here, of the following type:
XFX Core Edition Radeon HD 7870 GHz Edition DirectX 11 FX-787A-CNFC Video Card - Newegg.com

The new PC build I'm having problems with uses an MSI B550-A PRO motherboard:
MSI PRO B550-A PRO AM4 ATX AMD Motherboard - Newegg.com

Could I try plugging in my old AMD GPU into this new motherboard to see if that gets the new PC working? How can I check compatibility of the new motherboard and the old GPU?

I tried putting them into PC Parts picker and it didn't list any compatibility issues, but how can I check that motherboard & GPU myself to be sure?

https://pcpartpicker.com/list/x9PXBc

 

[kurdtnz]

Did you update the BIOS on your motherboard for the 5600X?

 

[rolli59]

The old GPU is compatible.

 

[MarsColonist]

Did you update the BIOS on your motherboard for the 5600X?

Yes. And it seems to work... on those days I get lucky and the GPU plugs for the monitor cables seem to be working right. I can get into Windows then, and do all the normal things you would do with a PC.