I think it’s a memory issue, most likely due to the sorry state of my Windows installation. Need to knock off the lazy and wipe it, but it’s pretty remarkable that it works as well as it does. I started with fresh Win7 and have survived upgrades to Win8 and Win10 in addition to the major feature updates that come now and again. I thought it was totally borked a few years back but some obscure automated tool managed to fix it.