It works! That was a strange interlude. Over the weekend my Internet connection, especially the upload speed, slowed to a crawl.
Let's bore everyone with details.
I've got a wireless connection between my computer and the router. The wireless connection between the router and my phone was just as slow, so the computer wasn't the problem. Plugging the ethernet cable directly into the computer didn't speed things up, but it did eliminate the router as the problem. That was good news because the two expensive items weren't the cause of the problem. The modem lights were blinking properly and resetting the modem didn't improve the situation.
Hmmm. What to do next? The only thing I could think of was to look at my ISP's web site. Muddling around there for a while wasn't very helpful but I eventually stumbled upon a page where I could test my connection speed. Downloading was fast but not only did my connection fail the upload test, it actually caused that test to crash. I reloaded the page and started the test again. This time both download and upload speeds were normal. Four more tests resulted in four more normal speeds.
I don't know if the test fixed the problem or if the service improvement coincided with the test, but the problem has been fixed for the time being.