Pi 4 Flicking HDMI

Hi there, my Pi 4 just arrived and the display output is flashing and flickering:

Has anyone seen anything like it? It’s got a 3A PSU and I can still ssh into the device so it’s not… not working, but can’t get a sensible HDMI output at all even after trying various config options and cables. So strange because it’s stable for a few seconds during the initial boot.

Any ideas, or is it defective? They’re quite pricey at the moment, so I need to get it going or send it back asap really :(

If you installed Bullseye, try Buster instead. There are a few glitches on the go with Bullseye.