I was having a similar issue a while back and thought the chain seemed plenty tight enough but it would drop on bumps or i could hear it skipping if i was really trying to climb, I just took it into my lbs and in like 5 minutes they tossed in a half link and all was solved. Haven't had an issue since. (not saying this is definitely the problem you have, just relating a similar situation. but it couldnt hurt to take it in. shrug.)