4K export poor quality

Any advice with regards to this Premiere Pro export issue would be greatly appreciated. 

I have exported a 7 minute video in 4K however the background of the exported  video seems to have a kind of shimmering effect and it's basically ruining the vide bcos it's not the crisp and clear 4K I expect. It's difficult to explain what it is, it's not blurry and doesn't appear to be pixellating, but I suppose it's like a shimmering or sparkling effect. 

The source and output settings are the same 3840 x 2160 at 60fps, target bitrate is set at 65, and I have tried both h.264 and HEVC h.265 codec but it's producing the same issue. I've watched a few YouTube videos on export settings and have copied the recommendations but the problem has persisted, 

I have exported a few other videos of late using the same settings without the same issue occurring. 

Please help.

Thanks in advance.