Premier XL4 frame rate drops

I have a Premiere XL4 (758) I bought in 2013, with Lifetime. I upgraded to a recommended model 3TB drive two years ago, and it's been fine. Earlier this year, I had a number of random reboots during playback and recording, so I ran the various hardware disk diagnostics, and it seem to have stopped rebooting.

Recently, I've noticed that every once in a while, the frame rate on playback of my recordings drops to something less than 24 fps, more like 12. Audio is fine, and does not stutter...

Premier XL4 frame rate drops

Premier XL4 frame rate drops