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?
I would start by finding out which version(s) of flash (or flash alternatives are installed).
$ dpkg -l | egrep "flash|gnash|swf"
Also check if was installed manually (some 64-bit users do this):
$ find ~/.mozilla | grep -i flashplugin.so
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.
I have figured out over time that flash tends to have problems it crashes and refuses to play certain videos on youtube what i done to fix this issue several months ago was sudo apt-get install gnash
installing Gnash becomes default over flash