Thanks for the links, Dust! In a way, it's interesting to know that others have the same issue, but also vexing that we don't have a clear root cause. I've tried typing on both my 1" felt pad and my usual rubber bar mats, and the problem persists (though, maybe slightly less frequently on the felt pad).

I'll try another cleaning pass on the escapement and u-bar (going with the theory from Ted Munk's site that working the grime out helps). I'll also try adjusting the trip point - maybe I can find a more forgiving position. I may look at the adjustments on the escapement, especially since I almost completely disassembled it. Who knows how close to accurate it is now.

I'll report back if I find any improvement. I really like this machine, so I'm going to give it my (current) best. 

If all else fails, I can take it over to Bremerton Office Machines and see if Paul can help me out.