Welcome to the forums! I hope you stick around, it's a nice place to hang out.
To figure out what the problem is, a bit more information would be helpful. When not using the YouTube app, how are you viewing videos? Through the web browser (either the default one or another one), or are they some you put on the SD card? If the second option, what program are you using to view them, and what format are the videos in?
Thanks ahead of time for answering - I know it's quite a few questions, but it'll really help me (or anyone else) figure out exactly what's going on.