Again...
The problems that people are seeing are completely related to memory (RAM). Mac Pro systems are a completely difference platform for Apple. Please remember this!
The manufacturer, the sizes and placement of RAM are extremely crucial in Mac Pro using Final Cut Studio. I, too, had this issue when I first purchased my Mac Pro BUT contacting the vendor I bought the computer from (ProMax) and they too discovered this issue. ProMax completely replaced my memory with Kensington memory and I have had no issues with memory since.
Please read this paper from Apple on this...
http://docs.info.apple.com/article.html?artnum=304492
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Wayne Carey
Schazam Productions
www.schazamproductions.com
http://blogs.creativecow.net/waynecarey