Thanks for the replies. I've tested my site and my cart using other computers on other networks, and it all works fine.
I had an order for one of the widgets today for the first time in three weeks, and I've been having orders for other brands of widgets all along. It's just this one line of widgets that's just gone down the tubes. I'm even increasing my AdWords spending on it, but no go.
I've also checked on competitors, and I haven't found one of Google's page one retailers who are lower in price than I am. (I was once on page one for everything, but that ended 2/24/2011).