Can you help diagnose this television fault?

Television has distorted/black image for several minutes after powering on. Goes away after a bit, although this time is getting longer each time.

Distortion isn’t purely horizontal or vertical, and only happens on part of the screen (when it goes black it is all of it though). I’m trying to upload a video showing exactly what it does.

Happens even when using the OS so it’s not related to any inputs, that’s all I can figure out.

I’d be very grateful for any help.