iMovie MPEG-4 import problem

I'm recording Xbox 360 with a Hauppauge HD-PVR on a Windows Vista PC. The Haup. software save the files as MPEG-4. These files play fine on my PC and on my iMac in Quicktime.

When I import the MPEG-4 file into iMovie I have audio but no video.

Any suggestions?

I've had to use iSkysoft video converter to convert the file to MPEG-4 again. Then the file works fine in iMovie.

Why isn't the orginal MPEG-4 working in iMovie? Feels like a waste of time to have run the orginal MPEG-4 through a converter to create a new MPEG-4 or other file to work in iMovie.