Youtube videos stop suddenly

SOURCE: Streaming video on the internet

Now there are many streaming video recorder. It can download streaming video offline.

I use this one: http://www.flash-on-tv.com/streaming-video-recorder.html

You can have a try.

SOURCE: I have a mac air and I am having volume problem.

The problem is that the headphone jack for you Macbook Air is a copy Analog (standard headphones) and Optical (toslink) jack. The jack has a small microswitch that detects when the cable inserted into the jack is a metal analog connector or a fiber optic toslink cable. The reason your seeing the "NOT ALLOWED" sign (circle with a slash through it) is because you cannot not adjust the volume on the sound when the output is in DIGITAL MODE. To verify this, drop down the jack cover on the side of your macbook air and place your index directly in-front of the headphone jack about 1/2" away from it. Is you finger glowing red? That's because the fiber optic LED is turned on and Mac OS X thinks that there is a toslink cable inserted into the jack.
This is a known hardware problem with that type of combo-jack and affects a lot of other Macbooks, as well.
If your Macbook Air is still covered by Apple Care, take it into an Apple store and have them replace the jack. I know this is a pain, but it is the correct answer.
If your Macbook Air is NO LONGER UNDER WARRANTY, you can attempt to fix this yourself, but be warned, you could break stuff!
Here is a thread at the Apple support forums discussing this issue: http://discussions.apple.com/thread.jspa?messageID=3255575
The above link is good background reading and will help you understand the issue further. The short answer is that many people have inserted a toothpick (other similarly sized NON METAL object) into the headphone jack, wiggled around until the microswitch is clicked off, then normal audio resumes. My 2009 Macbook Pro 17" has the same issue, but I have found that I can take a regular pair of headphones, insert the jack, wiggle it, remove it, reinsert, remove it....and repeat this process until the switch gets toggled. It's annoying, but I have to do this until I get home and can send it back to Apple for warranty work.