Question Issues with a new computer

[Centriole]

I've recently put together a new system containing, primarily, a Ryzen 5 5600x and MSI b550m pro vdh-wifi.

When I originally got it all together, I had used an old PSU I've had laying around for a while, and I was able to boot, however trying to boot off of a boot stick caused the system to immediately restart. I also had random restarts while sitting idle in BIOS for a little while.
My first thought was an issue with the old PSU, so I swapped my current system's definitely functional PSU in and tested it to get flickering on all the LEDs periodically, but the CPU LED was stuck lit, which had not been occurring previously.
I plugged the old PSU back in to make sure it still worked the same off of that, and it did.
Eventually I found that if I only plug in the left 4 plug into my board's CPU power 8 plug, it booted on to the same results as I was having with the previous PSU.

At this point, there are still a few things I can try, but I'm worried the issue is a defective board. Any advice?

 

[Lutfij]

Welcome to the forums, newcomer!

When posting a thread of troubleshooting nature, it's customary to include your full system's specs. Please list the specs to your build like so:
CPU:
CPU cooler:
Motherboard:
Ram:
SSD/HDD:
GPU:
PSU:
Chassis:
OS:
Monitor:
include the age of the PSU, apart from it's make and model.

trying to boot off of a boot stick caused the system to immediately restart
Did you breadboard the system or was it assembled in the case?

 

[Centriole]

Ryzen 5 5600x
ID-Cooling Frostflow X 240
MSI b550m Pro-VDH Wifi
Silicon Power DDR4 8x2 16GB 3200MHz
Samsung Evo 980
(Probably about a 5 year old GPU) Nvidia GeForce GTX 970
First PSU, maybe 10 years old: Corsair TX650w
Second PSU, about 4 years old: EVGA Supernova 750 G2
DeepCool CH370 Case

I've tried booting up a Ubuntu stick that I have and I've tried booting off of a Lubuntu ISO, to the same results in either case, haven't tried a Windows stick yet but I will today

I've had it in varying states of undress, but it's been in the case the whole time. It doesn't seem like it's an issue of shorting out, given how consistently I can reproduce the issue.