please state the type of media you are having issues with such as
smb shares (connection to PC may be the issue)
videos via USB (your USB drive may be failing or may not work properly with your wii http://wiki.gbatemp.net/wiki/USB_Devices_Compatibility_List)
videos via SD (your SD card may be failing, probably not but possible)
online media (your internet connection may be weak or your modem may be failing)
its possible your copy of wiimc is corrupted but as you said you reverted to a older version which i assume means you deleted wiimc and copied a different copy to your SD card.
if all your video is local its faintly possible your videos may not be 100% compatible with wiimc or they could be corrupted but i assume they play fine on your PC so they probably aren’t corrupted and since they worked before compatibility may not be it either.
So to be able to help you will need to provide far more details on what exactly you are doing.
preferably step by step details.