Some clips in my timeline have slipped or are no longer avaiable

I finished an edit two days ago, shut down my computer and detached the external video drives.  Today, I reconnected and started up and now the same timeline is showing some clips with white stripes.  Playing back those portions, it looks like the clip is either "devoid" of video (there's a clip there, albeit, white striped, but what I'm seeing in playback is the clip from the video track underneath) or it's slipped by some unknown amount. It's hard tell sometimes exactly what's happening.

This affects seemingly random clips throughout the sequence, with no obvious pattern, consistencey, or connection between them. The source video files are all there and have never been moved, renamed, etc. I just shut down and restarted - nothing else changed.  Also, not all clips associated with a video file are affected.  I probably have dozens of clips cut from that vieo file, but only one of the clips is doing this.  Some with another video file - maybe one or two clips affected, the rest are fine.

This has happened before, but I wasn't so diligent about observing the details and assumed it was something I did. That is definitely not the case here.

I looked up some solutions and there's a lot of talk about clearing files, relinking media, etc.  I've tried these things and it's done absoutely nothing

What's going on here? How do I keep my timelines from currupting? If I can't go back and reliably make a change, or add a title to something I just edited a few days ago, we have big problems!