I've recently encountered a problem when hammering it up hills. I have a compact crank and 12-25 out back. When hammer and tong in the 34T and somewhere near the top of the cassette it appears as if the chain is dropping to the next lowest chainring of the cassette. This is not a very pleasant feeling as it is a great unexpected jolt which usually results in my shoes becoming unclipped and me almost getting knackered. Everything shifts fine when on the flats so i don't think it has anything to do with the derailieur. Could it be the angle the chain is on or something else?