FCPX suddenly started dropping frames -- why?

Hi, Friends!

I've worked on many projects over the past few months and just yesterday FCPX (v10.4.7) started dropping frames as I was reviewing my progress. I've reviewed my progress a few times this week, but this issue just started yesterday. I import all the clips onto my iMac, do the work there, then export the finished video to an external drive. My iMac has 435 GB out of 1 TB available, so I don't think storage is the issue for a five-and-a-half minute video in 1920x1080 resolution. I've tried closing all open applications and rebooting the computer, then loading only FCPX, but it still drops frames. The head-scratcher for me is that it only appears to be consistently dropping frames on three sequential clips (one of which is a Fade to Color transition, the others are 4- and 2-second clips). Now I'm worried that the frames have been permanently dropped. What would/could cause this, and what can I do to fix it (and prevent it from happening again)?

Thanks!

Chris