External monitor blinking on and off

Thanks for that info Joefromstockport.

If this is happening on two different monitors, it would appear the issue lies in the Mac itself. With that being said, we may also be looking at possible software issues causing the problem. Let's test in a test user to see if the same issue occurs. That will tell us if this is a system wide issue. Here is how to manage and add new users to your Mac to test.

Change Users & Groups settings on Mac - Apple Support

Also, let's also try the following:

1. Start to safe mode, then restart to your regular user and test again - How to use safe mode on your Mac - Apple Support
2. Reset the SMC on your Mac - How to reset the SMC of your Mac - Apple Support

Let us know if any of these steps help, and we'll go from there.

Cheers!

Hello Joefromstockport,

Welcome to Apple Support Communities.

 Check the ports on your Mac to find out whether you need an adapter. If you're using a USB-C or Thunderbolt adapter to connect your display, the adapter must be compliant with DisplayPort Alt Mode, Thunderbolt 3, or Thunderbolt 4.

If you use an external keyboard and mouse with your Mac notebook, you can close the built-in display after you connect your external display. You might have to press a key or click your mouse to wake the external display. To use your Mac with the display closed, you need to connect your power adapter or connect an external display that also charges your Mac, such as the Apple Studio Display or 23.7-inch LG UltraFine 4K Display.

If your external display doesn't turn on after you connect it to your Mac, connect your display while your Mac is off, then wake or turn on your Mac.

Connect a display to your Mac

If you're using a Mac with the M1 or M2 chip:

On MacBook Air, 13-inch MacBook Pro, iMac, and Mac mini, you can connect one external display using either of the Thunderbolt / USB 4 ports. On iMac, connect your display to either of the ports with the Thunderbolt symbol .

As explained in the same link above.

Also, In order to isolate this issue further, we'd recommend connecting your external monitor with another Mac or computer to see if you experience the same behavior.

While the following support article is for help if your external display isn't detected or appears blurry, the troubleshooting suggestions may also apply to what you're experiencing. Please try all the steps in this link:

If your external display isn't detected or appears blurry when connected to your Mac

We hope this information is useful. Kind regards.