I too have this problem with iMovie. Searching the web for fellow sufferers I find lots of discussion group posts about this issue, which no-one ever replies to. Surely to goodness someone out there knows the answer?
And another thing ...using iMovie on my desktop G4 gives me hellish problems with pixellation on titles and transitions. Another Internet search suggests, again, that many people have this problem and, again, no-one has any answers.
Is iMovie just fatally buggy? Why doesn't everyone have this problem with it - and why doesn't anyone know how to help the folk who do?
Very odd.