HDMI display started flickering

I’m running Kodi 16.something (with retropie) on a raspberry pi3

It’s linked up to my TV with a HDMI cable, and also has an external hard drive plugged in for all my media.

This has been running with no issues for about 9 months.

I turned it on yesterday. I have a video splashscreen. That played and then kodi loaded up but then the display kept flickering on and off the HDMI, like it was displaying kodi one second, then looking like it wanted to readjust to optimal HDMI settings but couldn’t quite find it. Sometimes, it would stick on the no HDMI input.

So far I have tried (in vain);

1. Changing HDMI cables
2. Changing HDMI ports on the TV
3. Unplugging everything from the USB so it only has the HDMI and the power source connected to it
4. Changing the power supply
5. Changing the config.txt to uncomment out the config_hdmi_boost=4

Not sure what else to try (apart from wiping the SD card clean and starting from scratch.
This has only started recently and not as a result to any big changes I made to my pi.

Any ideas on what I can do to get it working again?