I'm working on a project which has two different versions of one of the elements in the scene. One of the instances is in a wide-shot which has been zoomed in significantly. Because of the lack of detail in the original wide shot, I've overlaid a closeup picture of this same object over the top of it to provide a clearer image. Both the wide shot and the close-up overlay are png stills. The overlay has an alpha channel so that only the overlaid object shows on top of the indistinct version in the wide shot.

In addition, I've punched holes in both tracks using masks, so that portions of a lower track show through in the appropriate position. Neither of the masks is keyframed - the holes are constant in size and are present throughout the duration of the clip.

When I play the timeline, or single-frame through it with the arrow keys, everything is lined up perfectly. Upon rendering, however, one of the tracks shifts slightly, but perceptibly. I eventually found out that this track moves every time I render, whether it's the entire track or just a small portion of it. If a small loop in the middle of the project is pre-rendered, the shift is observed for the duration of the pre-rendered loop. The shift takes place at the first frame of the rendered portion and goes away at the first frame after it.

Any ideas why this might be happening, and how to fix it?