Intermittent bad picture quality

[Buzzcut]

Hi, I've noticed that the picture quality on videos being played on my SM player has gone down lately. The videos play with quite a bit of visual noise, more noticeable in darker areas I think. The thing is, it will suddenly change to a lovely clear picture for some seconds and inevitably goes back to the lesser quality. This can change in a single frame - the pictures might illustrate. The file plays clearly throughout when on WMP, so it's not the file & the files are HD.
If you look at picture 1, look at the degradation under the woman's nose and lip for example and on the guy's cheek. In the screenshot of the very next frame [pic 2], the picture clears up to a more natural look. Any ideas? Thanks in advance.

I have no filters applied, no processing.
I've deleted the configuration file to "reset" the personalised settings that I had and I have reinstalled the program. Strange one.

[rvm]

Try a different video output in preferences -> general -> video.