PlayStation 4 Slim + Sony TV - no picture

Righto, so super strange one.

PS4 Slim with a Sony TV (5-6 years old, 55" model.... can't see specific model number as it's on the wall, but will find out shortly). Then there is a Yamaha HT amp.

The set up has been working fine for the past few months, but now (not played it for a week or so) we have no picture. And the TV just acts like someone is unplugging/plugging an HDMI in and out constantly.

Settings on PS4 I mucked around with:- Fixing resolution to 1080p- Turning off HDCP

Physical troubleshooting:

Removed the HT amp from the equation. So PS4 plugged directly in to TV - no video or sound - At this point tried another input - also tried another HDMI cable (several actually)

Connected PS4 to PC monitor. Works perfectly, picture and sound automatically as expected.

So then connected PS4 back to HT amp, connected PC monitor to output of HT amp. Works perfectly. Sound through HT speakers and video on PC monitor.

Now the other weird thing is that other devices are working fine on the TV... Raspberry Pi, Chromecast, laptops etc. All work fine connected directly to TV and via HT amp.

So before I go out and buy a new TV, what am I missing?

But the biggest issue of all is I suck because the partner can't play Spyro.