500D 720p 24 fps stops recording after 40 sec

[cyclonite]

Hi Guys first post
I did a search but could find nothing here or on the big G.
I am trying to record 720p @ 24 fps on a 500D but it stops recording after 40 sec.
I have a class 10 card and it records fine @ 720p/30fps and 1080 @ 20 fps for the full 4 gigs.

Any idea's what i'm doing wrong?
Thanks
Brian

[g3gg0]

did you disable audio recording?

[cyclonite]

Perfect, thanks very much