Rendered Video Repeats Frames

The problem started subtle, just a few frames per video but now it seems to be happening every 20 frames or so. As an example, the rendered video plays frame 20, then frame 19, then frame 21.

Playback is smooth in the sequencer and smooth when an image sequence (JPG) is rendered but when I encode as QuickTime the frames start stuttering.

Anyone else having this issue?