Rendering from XML clips produces jumpy video

I've been using MLT's in= out= function and exporting to an XML consumer to create pre-trimmed clips. After importing the individual XML files into Kdenlive and dragging them to the timeline, I can then apply effects and export as a complete film.

So far this is the only method I've found of translating other systems' EDLs to Kdenlive (using Perl) without having to understand all the intricacies of the Kdenlive project XML file.

The problem is that in using this method, the exported video is jerky and jumpy, whereas if I put the video clips themselves (instead of the video clips enclosed in XML) on the timeline and export, the exported video looks fine.

Is there an explanation and solution for this?

Many thanks in advance.