David…I tried your suggestion with no luck.
What was happening is the timeline would play OK for 2 seconds or so then freeze for half or a full second and that would repeat. It got worse as the playhead moved down the timeline.
So I began to think maybe it isn’t a frame rate error even if reducing the framerate to half got rid of the problem. So I began shutting tabs. I had about 10 open because the timeline was split into ten different sequences. So I shut them all except the one I was working on. I also shut the effects tab and a couple of tabs for music and graphics. And that seems to have solved the problem. I now have smooth playback.
So do you think this was some kind of memory problem? Too many open tabs at once draining memory?