OK, so 3 weeks ago I replied that I had followed your instructions, but that it didn't seem any different. However, in the weeks since, it hasn't come loose again! So what you recommended did the trick. Thanks!
I will say, though, that the there is a "little" wiggle in the handlebars. But that seems to be a constant. That is, it's not getting progressively looser and needing continual tightening, as it was before. It would be good to get that little wiggle out, but it doesn't seem to be affecting anything. And I don't even notice it unless I purposely go to the handlebars and try to wiggle them. So it's not really a big deal.
So thanks again! Sorry I was skeptical at first.
FYI, I had e-mailed Rad tech support, but they couldn't figure out what the problem was, so they sent me a new stem, to see if it worked any better. Before the new stem arrived I applied your solution, so I never opened the package. So I'm going to contact them and let them know your steps, as well return the stem.
Thanks again!
Neil