Stuttering after import from Xfire Video

I use xfire to record some of my gaming sessions on my PC. The video is captured at 60 fps at the same resulution of my monitor 1680x1050. It plays fine in windows. Xfire is then used to convert the video into .mp4. I then transfer that file to my imac.

The imac can play the video without any problems in quicktime or vlc. Once I import the video into iMovie 11, it stutters when I play it or upload it to youtube. Here is an example of the stuttering.

http://www.youtube.com/watch?v=vD3FPXDC2l8

Any help resolving this issue would be greatly appreciated.