True to my word, I would now like to share the solution. Yesterday, I tried the very first thing that Luis had suggested — trashing Preferences. Many video forums list this as one of the first things to try in order to solve playback and other performance issues. At first blush, this seemed to work somewhat — playback was still intermittently choppy but not quite as bad as it had been. I continued work on the project at hand and was dismayed and alarmed when within a few hours of trashing the Preferences as Luis suggested, performance got dramatically worse. FCPX crashed repeatedly over the course of the next 24 hours, sometimes five or six times an hour — mostly when image stabilization was applied, but also during routine editing. I was considering exporting the project to another editing system when something remarkable happened: Apple issued a Supplemental Update to 10.15.5 about an hour ago. In my original post here, I opined that the 10.15.5 release had been the cause of my choppy playback issues — so I immediately installed the Supplemental Update. Bingo! Problem solved. No more crashes, no choppy playback. As usual, the whole fiasco was my fault — I have an ironclad rule to never upgrade system software until at least a month or two after it is released, but I broke it in this case. FCPX seems to suffer more than most apps from upgrade-itis. Bottom line, there was no need to buy special diagnostic software, and no way that any conventional fixes would have worked — once Apple released the patch for their upgrade, the problem was solved.
