Jumpy Transitions

[ibenz at mac.com]

Hi
I'm using iMovie 3.0.2 and here is my problem.
First when I try and preview a transition I click my mouse on the 
'Preview' button which lights up for a couple of seconds and then just 
reverts back to its normal state, without showing me the transition 
preview.
Secondly, when I have put a transition in the timeline, the movie plays 
smoothly until it reaches the transition and then sort of stumbles for 
a couple of frames before moving onto the next clip.
I have just upgraded from iMovie 2 and never had these problems.
Help me, I'm going mad !
Kind regards
Neil