Recently when I'm pedalling up a steep hill, in the smallest chain ring (of 3) in front, the chain gets stuck between the 2nd and 3rd chain rings in the front. The strange thing is that its not when I'm shifting, am already in the small chain ring and going at modest pace, no special load. previously have had very occasional chain suck while shifting, this is not that.

When this happens, it seems that the chain has skipped off the back cog, down from the 2nd or third largest to the fourth largest. perhaps then its too loose and gets caught up in front? but do not know why it would skip off the back.

chain and cog are used, maybe chain is stretched and need to get new one. recently cleaned chain and cassette and front ring extensively, maybe without all the crud it can slip off more easily?

bike: rivendell, set up for touring.

anyone had similar problem or any advice?