Bizarre predefined character replacement

I’ve noticed that in posted comments on my site, WordPress automatically was automatically replacing the apostrophe and quotation mark with [amp]#39[semicolon] and [amp]#34[semicolon] respectively. That was working fine until the last couple of days. Now, I’ve discovered that almost every comment on the site (even old ones) have had the apostrophes replaced with "&apos" (or sometimes "'") and the quotation marks replaced with " &quot ".

The big problem is that the predefined character codes are NOT being replaced by actual characters when the comments are displaced, either on the front end or the back end. So, now a lot of comments look like:

“So, I read this interesting book and the author"'"s thesis is that something has gone terribly wrong with the code. I don"amp"apos;"t know what to do.”

I’m not sure what caused this or how to fix it; I haven’t been able to figure out any way to write a search and replace query or use Search Regex to even find the [amp]apos[semicolon] strings in the database table, much less replace them with something not broken.

I DO have a function that is designed to strip HTML from comments, but that function has been in place now for almost two years and it never caused this problem before.

Any ideas? I’m baffled and over my head.

Thanks!