Pardon my stupid. I'm having some issues with my iMovie HD 6 when it comes to rendering clips with some of the special effects*, specifically the Video FX Fast/Slow/Reverse effect. Every time I try it, it renders the clip all the way (the red line completes and goes away) and then the clip goes black and "disappears" - when I try to play it, it jumps immediately to the clip after it. There's still an icon in the project bar, but the content of the clip and is gone.

*Some of the special effects work fine, it's primarily this one that I'm having issue with.

I'm sure I'm just doing something ignorant, and normally I'd just play around with it until I stumble on the answer. But the project I'm working on has a deadline and I'm already behind on account of other technical difficulties. Any help would be greatly appreciated. Thanks very much!

(x posted to macrumours; apologies)