Mail on 10.4.11 OSX losing Inbox emails

[Chris S]

Hi there, the Mac Mail 2.1.2 app on my G4 Power Mac running 10.4.11 has suddenly gone weird.
Some messages from the Inbox drop themselves into the trash or other folders then when I quit/shut down they have gone from all folders. Annoying when I have not answered them.
Tried an idea with some free software (P..... something?) to uninstall Mail app from OSX and then reinstall the 2.0 Mail which came with the original discs but it still did the weird stuff, dropping some emails.

As I shift between trash and inbox more mails will drop in the trash which suddenly reads many more trashed emails than there really are

Done the usual repair perms and some other stuff I've since forgotten

The same 2.1.1 Mail with same settings works fine on my much less used Macbook so I take it changing the User account (?? not sure what that is/how to do) or email settings is not the problem.

Moved Mail folder from Library onto Desktop and reopened but still did weirdness (and did not appear as a new folder in Library)

Never has a virus on my Mac in 13 years or more but that is what it feels like. I thought we were immune. Any ideas short of changing up to 10.5?

I hope you can help, thanks

Chris S