I haven't found any other discussion of this but here's my experience:
I've got a build with 4 layers of video and one of the layers doesn't last long enough. When I put a Time Scrub filter on it to lengthen the duration, it causes the bottom most layer to jump around in time. I've noticed from older posts that time-remapping and/or slo-mo is not very well supoorted in Motion. Is that what's happening here?
Final Cut Studio
DP 1.8 G5
2.5 GB ram
Now maybe I'm new to Motion, but I don't quite understand why, when I add a filter (in this case Desaturate) to a layer that is reduced in size over another layer, the video from the bottom layer goes away and a full size, non-Desaturated version of the second layer is in its place - even though according to the timeline, nothing is amiss...