An example of the "gobbledegook" might be useful. You might be seeing an encoding issue, though I don't understand why it would appear after some time.

It seems rather more likely to me that your message index is corrupted, so you're seeing parts of another messages, and out of context, Thunderbird doesn't know how to decipher them properly.

However, given that you're seeing this in messages that are some days old, your usage pattern is rather different to mine. I don't keep old messages in the Inbox; I file them if they're useful, or delete them.

I think you need to repair your Inbox, but if you're in the habit of keeping old messages there, you have a risk of losing messages if you just repair it. And another question: do you allow it to compact often? A combination of not compacting and repairing can erase a lot of messages. :-(

Before doing anything, I think you should make a backup copy of your profile, then at the very least, you can get back to where you are now.

https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data

To repair a folder, select it, right-click it, select Properties and then Repair Folder.

Here's an example:

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I have backed up my profile.

I always compact whenever requested, but I don't delete messages. I have now been through and deleted lots of messages.

I've clicked on 'repair folder' and it seems to have done the trick - Thanks!