Kinda stumbled across one earlier this year; initially thought it was going to be a flip but I fell in love. Just took it out yesterday in fact.
Untitled by
Eat More Plants1, on Flickr
FWIW I have a brand-new 28t 6-speed on there (brand new chain too) and experience some similar symptoms. I have fiddled with the b-screw, the low gear stop, and I even checked the derailleur hanger alignment, can't quite figure it out. At least for the time being I'm chalking it up to one of two things: the Shimano 600 derailleur is a bit more finicky about tooth count than the conventional wisdom suggests (I had the original 600 freewheel but it was like a 24t or 25t or something, which was a tough slog on hills with a 52-42 up front), or it's simply frame flex under load that's tweaking the alignment down there. I say that because it's way more noticeable when I'm standing, trying to get up a really steep hill. Sit-and-spin sort of stuff doesn't seem to trigger it. Let us know if you figure it out!