Internet connection speed was fine up until about a month ago, when things started to get colder. Now things are barely dial-up speed, and getting worse. Is this a reasonable cause and effect, or should I look for other causes?
In short: Yes.
Not knowing your circumstances though, call your ISP tech to check the cables, see if there's anything loose or not protected. But many reports indicate some problems when it's really cold out there.
As the weather gets colder, there is often more condensation. If there are junction boxes that are not weatherproofed or cables that are damaged, then yes, the weather cooling off can have an effect on your connection. Getting your ISP to do anything about it might be very difficult as it will be an intermittent issue, and usually when its coldest or wettest, the tech will not be there to test it.
Needed to re-cycle my router. That fixed it.
You all got upvotes for your answer, though. Thanks for your help!