In search of a home for Thunderbird

Needless to say, this is not remotely normal. I feel like you'd be better off figuring out what exactly is so desperately and uniquely wrong with your machine than just trying to work around the symptoms.

Best guess, some kind of seriously shonky third-party NIC driver, possibly one designed for a different OS than the one you're actually using (and thus with bugs that didn't get triggered when it was written in 2001, or whatever, but do now) or even different hardware.

I would definitely try the latest version of the relevant driver from the chip manufacturer (as opposed to the board manufacturer, so if it's a DodgyCorp NIC or MB with a Realtek chip for example, I'd go to Realtek), and I might also try a slightly older but not prehistoric version. Unless it's Broadcom of course, in which case I'd use a hammer.

Of course it could actually be hardware.