Screen stretches, turns green/purple when playing video and cursor disappears

[Pindi7]

Hi,

I recently set up a 4k tv to connect through an av receiver to PC (running a gtx 780) through hdmi. Everything seems to work fine except for when I play a video, mainly MKVs. It'll play just fine but when the mouse cursor disappears through inactivity, the screen suddenly stretches/distorts and turns green/purple. Moving the mouse brings the normal picture back.

I thought it was a MPC-HC (kawaii) issue but it happens on windows media player too. It's only started happening when I switched to 4k and it doesn't happen when it's not full screen (earlier it didn't happen at all so not sure what's different now).

Can anyone help?

Thanks

 

[Pindi7]

switching back to 1080p doesn't help. I've noticed that the nvidia 'press alt z to share your gameplay' banner pops up now on video when it didn't before too, not sure if that's helpful