I recently bought a 1984 trek 610 with a shimano 600 groupset. The rear cogs were pretty worn so I replaced the freewheel with a 13-28 hyperglide. The chain has been dropping off of the chainwheel intermittently. I've done some careful shifting and it seems to happen when I'm shifting up to the bigger chainwheel. The chain falls off on the outer side, so in between the large chainwheel and the crank, but held up by the front derailer. Upon further inspection, it appears as if my rear derailer may be bent,. Not the whole thing, however, just the jockey pulley and perhaps the linkage between the jockey pulley and tension pulley. I'm guessing that the problem lies either with front derailer adjustment, or the possibly bent rear derailer.
Here's an ad from the previous owner: http://slo.craigslist.org/bik/1440640877.html
And a ****ty cell phone picture of the rear derailer: http://i154.photobucket.com/albums/s...y/derailer.jpg