VLC Blocky compression artifacts on .mp4 only

[fl00d]

So VLC displays these blocky artifacts when playing .mp4 files. Other file formats seem to be fine. WMV is flawless, for example, by comparison.

The artifacts get worse and worse until *I think* a key frame comes around and then the video cleans up, but then gradually degrades again in cycles of about 5-10 seconds.

Windows 10, Intel core i7, NVidia GTX970, current drivers (problem has existed across many divers)

Anyone else have this problem?