Serious IPv6 performance problems

I'm in the Pensacola/Destin area, and until about six weeks ago everything was wonderful.

For IPv4, it still is.

However, for IPv6 it definitely is not.

It's not consistently bad however.  I run some large transfers down to my PC (software updates and such) and many of them are fine and run at full speed.

But I also have a FreeBSD machine here, and it gets updates (source) from svn.freebsd.org, which does a geographic lookup.  To this site I never get a good transfer since the problem started happening.  I'll get three, four or five files into it and then transfers go to ~8kbps and stay there, eventually timing out.

I've pulled the same update from a Colo machine that I have in NY, and it runs at full speed without problems.

There are also intermittent IPv6 only performance problems going to other sites.

This all started right around the first of November; prior to that it's been fine.  I've reset everything (including the cable modem) a few times, which forces a new IPv6 number to be assigned (you don't get the same one as you usually do for IPv4); some are a bit better and some worse, but none run at full speed.

Any idea what's going on here?  IPv6 is supposedly the future but at least on Cox at the moment the performance is anywhere between unreliable and abysmal a good part of the time, while the very same box, sitting in the same place, on the same connection, if I shut off IPv6, runs like the blazing race horse it should.