49" KS7500/KS8500 vertical lines?

Just bought a used 49" KS8500 (KS7500 in Europe) to use as a PC monitor. It worked great at the start but then I got vertical lines all over the screen.
At first I thought it was a bad HDMI cable or connection to the One Connect Box. But even when I disconnect the box I have the same lines. They also appear over the TVs overlay so it can't be the HDMI cable or connection? Using a brand new certified 2.1 cable.

Weird thing is sometimes it can work great for a while without this issue, then it comes back again.

There is no warranty on this TV and I really want to fix it. Is this a known issue and are there any parts you can replace to fix this?