Here is the link to your video,
[video=youtube_share;fRoeqN5HY44]http://youtu.be/fRoeqN5HY44[/video]
To me it looks like the motor never really gets started. Is there any way to disconnect the main gear box from the rest of the drive train and try to throttle up? This will either show the main box is binding or the other end of the drive train is.
What did you change since the last time it was running fine?
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This must be it when it WAS working.
[video=youtube_share;L4lvQngtPAo]http://youtu.be/L4lvQngtPAo[/video]
You can hear ESC noise in this video that does not show up in your problem video.You may have some speed controller issues here.