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Question power issues, mb or psu?


Mar 9, 2018

I have a setup a few years old (running i7-2600), the PSU is an antec 550W, mb is gigabyte and the gpu is an HD9650.
to this there is an dell 24'' lcd ips screen attached.
I've decided to get a new screen, got an 27'' dell ips led (I think).
when I start the machine after the screen replacement, I see bios screen and the system reboots, it goes forever.
when I connect the screen back, I system boots ok.
I've long suspected he PSU is dying, the mb has two gpu pci lanes and in past I had to use the secondary lane because there system didn't booted with the gpu in primary (different cpu, older and less power consuming).
what makes me think it is the psu is that if I connect a usb hub (ac powered) with the 24'' screen, I get the same reboots.
afaics, all points to the PSU but I cannot shake the feeling the mb might be the issue.
I've tested the 27'' screen on a different system and it boots fine.
as I don't have free mb or psu to check so before I go out and start buying hw, I'd like to consult
with your guys, are there any other things I didn't done which I should do to better find the root cause of the issue?