We can't really say what's the problem from our long distance but I'd speculate that the chain is well worn. I also suspect thet the cogs and rings are so unevenly worn that only some combos are miss matched enough to produce classic chain/tooth skip. A visit to some one who knows about these things (like LBS) should get you a very quick and more accurate assessment. As you're finding out this can be more then just frustrating, it borders on a safety issue. Andy.