I recently bought a new iMac. Shiny! It has snow leopard 10.6.8 and Final Cut 6.0.6 has been a total nightmare on it.
My 4 year old mac book pro has been using plain old leopard with final cut 6.0.6 and has had no problems. NONE!
At first I thought the problems on my iMac may have been because I installed Red Giant Looks software. Some forums suggested this may cause problems with Final Cut. I have uninstalled Red Giant and every single file relating to it. No help at all.
I strongly suspect that Snow Leopard is the culprit.