Did you buy a really fast SD card? If not, then the card cannot write as fast as the information is coming in. So the buffer gets filled up and the video has to stop to write the video onto the card. Here is a forum post about the same problem your having, except with a Pentax K7 but should translate over the same. It seems if you want more video recording time then you'll want to look at getting a Sandisk Extreme III or faster.

http://forums.dpreview.com/forums/readflat.asp?forum=1036&message=33695727&q=video+minute&qf=m