You are exactly right, PCMT! I cannot believe it. The blog looks mostly fine once I eliminated that. (Ulf Petterson just posted the same suggestion on his Modern theme blog. There must be something in the water.)
There still seems to be a little weirdness with the display, but nothing compared to before. I will go through all my posts to find the culprit.
Thank you so much. You cannot believe how relieved I am. I thought I was going to have to start from scratch with a new theme.