Nikon D3100 upload issue?

Whenever I take a video longer than, per say, 4 or 5 minutes, on my nikon d3100, it acts strangely on the camera (i.e.freezes when passing by it, when trying to play it, sometimes it doesn't play) and whenever I upload it to the computer, it comes up as '0 bytes'. I tried reformatting the memory card and googled the hell out of the problem but found nothing. Anyone have a similar experience / solution?

For now, use a different memory card until you can try recovering what is on it using a program like Transcend's RecoveRx (free)

Use this tried and true method of copying image and video files to your computer

* remove the memory card from your camera
* insert it into the card reader on your computer
* use you computer skills to copy all the files to a new file folder on your computer named for the subject and date.
* once all the files have been copied, format the card using the format feature on your camera

Do this at the end of each shooting day.