If it’s a usb drive that you are watching the videos from, go here
http://www.wiimc.org/forum/viewtopic.php?f=4&t=1033
You would want to run the usb drive test program and submit the results. Also, it might be very useful to download previous versions of WiiMC and see if they work. I have found “sometimes” the automatic download of WiiMC is bad, and I need to grab a fresh zip copy and manually put it on the SD card (apps folder). It happens… 😕