Problems playing videos

When I open video files (mp4, mkv, avi) in default Ubuntu 17.10 app ("Videos") I get messy picture with little color squares and after few seconds it totally freezes my screen, so I must restart computer. Probably some codecs missing...

How to check what's wrong and fix it? Thank you.

Graphic card: Advanced Micro Devices, Inc. [AMD/ATI] RV730 PRO [Radeon HD 4650] ubuntu-restricted-extras installed system updated and upgraded

If seeing these symptoms with Totem the 1st. thing to check is whether gstreamer1.0-vaapi package is installed. If so removing it can take care of the problem. This command will suffice to check & if installed remove.

sudo apt-get purge gstreamer1.0-vaapi

There have been continuing issues regarding that plugin since 14.04, for the most part they've been resolved in 18.04 though some hardware could still experience the video degradation.