Rendered frame rate problems with imported clips

I am compositing together several video clips, plus some images that are alpha’ed over the video.

All strips that I import into Blender have a constant frame rate (I converted any that don’t). Everything is 25fps, and I’m rendering at 25fps, so it’s not the “different frame rate in input” file issue. The problem I’m having is that video that I record from my webcam sometimes renders at a too-fast frame rate. I haven’t figured out what things cause this to happen. Most of the time it seems to work; the audio and video stay in sync. Sometimes, however, for reasons I can’t figure out, when I come back from an image (or another video) having been alpha’ed over the primary video from the webcam, the video in the rendered output file is rocketing along at something like 2x the rate it should be, while the audio goes just fine.

Again, this is not the entire strip. It works for a while… then it goes off.

I have both done soft strip offsets to trim things of the edge, and used K to cut the video into substrips and move around. Often, things work even when I’ve done this. But sometimes, for reasons I can’t fathom, the rendered video shows the included video strip rocketing ahead too fast.

Does anybody have any idea what’s going on here? What can I do to keep this from happening? It’s driving me crazy.