Slow Motion choppy in FCP 6

In the past I have slowed clips down in fcp with no problem.

Now when I try to slow clips down and render, they come out all choppy.

I saw in a youtube video you can get smooth slow motion by:

(1) exporting clip into motion
(2) using the optical flow
(3) exporting out of motion as a quicktime video
(4) importing the quicktime video back into fcp.

I did 1 and 2 fine, but when I tried to do 3, motion won't let me.

I right clicked on the clip in fcp, sent it to motion, hit the inspector tab, clicked on the timing tab, set frame blending to Optical Flow, set speed to 50% and when I went to export it as a quicktime file it says "Alert an error occured while exporting the project audio".

My project doesn't have any audio.

Help!

Thanks,

sltdmd