Stalling playback

I run a Studio, & we are producing a series of session & demo tutorials. The material comes from a variety of sources (i.e., musicians!), & I edit them together in QuickTime Pro 7.6.4.

The resulting movies play back fine, but then when I export from them to create iPhone music videos, they stall on playback at the edits.

For example, a guitarist supplied two files in exactly the same format, & I simply spliced them together. No problem playing back the resulting movie. But the exported .mv4 version freezes when it reaches the splice.

This problem seems to occur whatever format the source material is in.

It's not just freezing on my Mac; we tested these videos on an iPod Touch as well; so it is a problem with the export.

Watching the timeline in iTunes is entertaining, since it seems to slip back after a certain point… it's as if all sense of time has been lost.

I've tried searching for similar problems, but so many people are posting problems with QuickTime in Snow Leopard that it's too hard wading through that all stuff…

Any ideas? — I'm hoping this is a well-known issue!