If your cpu were not supported you would not get any video.  Crash suggests system instability.

If BIOS is telling you that you have a 3900X and you paid for a 5900X, you should notify Dell ASAP that it appears they gave you the wrong CPU. That usually wouldn't cause the black screens though.  What AGESA are you on?  Something like CPUz can pull more detailed CPU info for your machine.

If bios or msinfo32 says so, then your cpu is the 3900x (Zen 2), not 5900x (Zen 3).

Re: Pc bought 10 weeks ago as per spec below and invoice: 5900x.

did you check the cpu spec on the day of delivery?

type your service tag in Dell support and verify your system spec on processor.

it would be shocking if system spec says 5900x but actual processor is 3900x.

There could be any number of reasons, software or hardware, for the black screens.  

What does event viewer show as far as critical errors.

Run the ePSA built in diagnostics, f12 on boot.

Also, try a clean install of the Nvidia drivers but try it with the one release prior to what you have, if you have the latest.  Otherwise, try updating to the latest direct from Nvidia for the 3080.

When entering the private service tag on the generic support page, it shows that Jonny purchased the following =

Could also be mis-reporting of cpuz, sysinfo, etc...

For example CpuZ it reports that the R10 has a 570 motherboard when it is a B550.. Would not surprise me if the cpu's mixed up as well.

To really be sure what CPU you have, I suggest downloading Ryzen Master - it should indicate the correct CPU installed.

"Checking in the Bios

It says I have a AMD RYSEN 9 3900X"

It would be highly unlikely for BIOs to misreport the CPU.  Plus the service tag info is also pulling 3900X, which confirms what BIOs is reporting. 

Re: To really be sure what CPU you have

correct that bios and service tag are two strong pieces of evidence that cpu is almost certainly 3900x.  

if still wonders about ? 5900x, verify with the original sales invoice to rule out a typo.

or resort to litmus test: remove heatsink, wipe off thermal paste, and take a picture.

Well, if you paid for a 5000 series you should receive a 5000 series, not a 3000 series.

It would come down to what you actually paid. 

Do they even have a BIOS on the R10 that works with the 5000 Ryzen series?

There have been other posts on this forum about the disappointing lack of future upgrade-ability to the 5000 series for the R10.

Just curious but what is the BIOS version (example: 1.08, 2.x.x) with that 3900X (that was supposed to be 5900x) ???

Yes, they do. There are 2 versions of the Ryzen R10. One with a factory Ryzen 3000 installed, one with a factory Ryzen 5000 installed.

The version with the Ryzen 3000 cannot be upgraded to a Ryzen 5000 due to lacking bios support for the Ryzen 5000 lines of CPU's.

Apparently the only difference between the  R10 versions is the bios (AGESA version to be more precise). I do say apparently, because I have not seen any official Dell answers explaining the difference(s).

If the AGESA version is the only difference, than the older version could receive a bios update in the future to make it the same as the new version. (And support both 3000 and 5000 Ryzen CPU's). However, there has been no official announcement from Dell on when, if ever, this will happen.

Currently the general consensus on this board and other boards seems to be that Dell is blocking the old R10 from being upgraded with a Ryzen 5000, by blocking bios updates with updated AGESA code.

It is also possible Dell is waiting on the USB bios update AMD is pushing out, which requires a new AGESA version, to update the old R10 with a new bios and open up Ryzen 5000 cpu upgrades.