Click one song in iTunes, but a different one plays!

I store all my music for iTunes on an external USB drive. Recently that drive was giving me error messages, saying I should back it up and reformat it ASAP.

I did that, although I had a few issues along the way, and I think it's messed something up now. For some songs, particularly episodes of my own podcasts (which I pull into iTunes to convert to a smaller MP3 and to add album art etc), if I click on them, some other song in my library starts playing. For a number of them, going to "View in Finder" shows a completely unrelated song.

I can just delete them, except I don't want to accidentally delete those songs! What should I do?

Also, iTunes has been running really slowly ever since I updated to El Capitan a couple weeks ago. I have a 2-year-old Mac Book Pro with 8 GB of memory.

I should say, the very recent podcast episodes (I mean my own shows imported into the music section, not referring to shows downloaded to the Podcasts section) in iTunes correspond to the correct files, but the ones from a couple weeks ago or more do not.

Also, when I say iTunes runs "slow", I mean lots and lots of spinning beachball. Even while a track is playing and I want to stop it and have to wait 30 seconds or more for the beachball to stop so I can click "stop"...

The latter sounds like a dying hard drive. Or you're running the library off a slow USB 2 external connection or something.

It sounds like the transfer from the dying drive may have corrupted some songs, but you can just "get info" on a given track and correct the bad information.

The slowness occurs regardless of which external hard drive I run my music off of in iTunes. I click stop and it takes 30 seconds to actually stop.

This started happening right after I upgraded to El Capitan.