I sent a long post to Actiontec through their support page describing the issue I'm seeing. I also mentioned that other people are having issues as well, though slightly different. We'll see where it goes.
I received a firmware update via email from Actiontech Tech Support in response to my email via their support page. The installation was smooth, and performance has been good for six days. My problem was a little different from some of those posted here in that it took one week (only two data points for this, however) for the speed to drop, and when it did drop it went to 14 Mbps in one direction. I will post again if it fails.