Thanks, Pierre,
So, I’ve reduced this down into the simplest steps I can think of. Still no luck. Running Premiere Pro CS4 4.2.1 on Mac OS 10.5.8:
1. Create a new project
2. Create a new sequence – DV NTSC Standard
3. Add clip to sequence – using SkateBoAM.mov from Twixtor tutorial
4. Add the clip again to the sequence for padding
5. Create a second sequence
6. Add first sequence to second sequence
7. Apply Twixtor 4.5 effect to first clip in first sequence
8. Change Output Control Speed to 50%
9. Playback second sequence – motion is indeed reduced by 50%, but at the point the padding was added, it begins playing from the beginning.
Could this have anything to do with it: the frame size of my sequence is 720 x 480. The frame size of the clip is 360 x 240. The moment I apply the Twixtor effect to the clip, it expands in the output display to 720 x 480. The padded clip remains at the original size.
Thanks.