Videos on YouTube load for a few seconds and then it shows that it has completely loaded but not really. The video plays for some seconds and then stops and doesn't load any more. What's going on?
There is an old bug with flash and sound buffer. Videos are usually loaded OK, but as soon as the sound buffer is full videos will pause (usually around 3-4 seconds).
Try skipping through video to check if it starts playing again for 3-4 seconds. If it does you have to upgrade your flash and ALSA (probably use newer ubuntu version), if it doesn't then check your available disk space.