External Monitor turning on and off

I have a MacBook Pro purchased in 2019.

I have attached an external monitor and after starting up the external monitor goes blank for 10 seconds comes back on for 1 second and then off again.

I have rebooted in safe mode and the external monitor stays on.

I am assuming that there is some setting that is making it turn on and off when booting up normally.

Anyone got any ideas where I should look.

Thanks