Thanks for the replies, everyone!

Out the door on my way to work this morning I did inspect the stays, there aren't any flaws I can see. I wasn't able to make it flex by hand (for what that's worth). The frame also does not seem to be bent at all, unless it's so subtle that I can't see it. I didn't have time to pull the wheel and check the skewers and the axle.

However, the PO did install pretty chubby, high-profile 27 x 1 1/4" tires rather than the 27x1" that would have come with the bike. And even when locked down, the hub seems to have some play in it. This problem was happening under effort when the chain was on the outer (smallest) sprocket on the back, so is it possible that this is aggravating whateve rdish is inherent in the hub? I had a 1980 Moto Grand Record back in the day, and I do remember that even with skinny racing tires, there was some art to placing the rear wheel properly after taking it off, to prevent the tire rubbing on the stays. It could be that with whatever flaws the hub is showing, plus the larger tires, the wheel just can't meet the slim tolerances for the frame. I have heard that the old maillard Helicomatics are bad about excessive dishing.

I will try pulling the tire and checking all the things that were mentioned, and I will also try putting on the rear tire from my Super Course on the Moto to see if there's a difference - thanks for the suggestion, Bianchigirl! I can't believe I didn't think about that (d'oh!) The gearing on the Raleigh is similar; the Moto is 13-26, the Raleigh might be one or two higher on either end, (don't remember off the top of my head) but not enough to have accustomed me to different gearing enough to make the difference.

The adventure continues. Thanks for the help, everybody, I'll keep you updated.