If you are downloading the clip using a manager, you don't need to watch it as well. What downloaders are you using?

i use youtube video downloader

Try again, but don't watch it until the download is done.

The download link may only be valid for a limited time because YouTube automatically converts the basic file on the fly to the requested format and makes that available, so if you pause the video then the link may have expired.

but problem occure in only big file like 100 mb. and above. and another fact is problem occure since 1 week. before it , it was work properly

There may be three possible reasons for this. 1.The server might have timed out after you have paused for quite a long time 2.The file you are downloading might not be a resumable content download. 3.Youtube policies change many times and they might have put some restrictions

4.The best alternative for downloading videos is https://addons.mozilla.org/en-us/firefox/addon/video-downloadhelper/ Try IDM and see if the link is resumable or not and please post your reply Thank You

my friend tell me this is main fault in firefox. he suggest me to use idm. and now i use idm which work nicely