I very much doubt it's your gear changes. I think you would know (lugging and chain chatter or whatever it's called in English) and adjust accordingly.
As others have already stated, it could be your idle is too low. Try anything between 1000 and 1100 rpms (if you have a rev counter, if not, listen to your adjustments, you'll know if it's too much). Anything over that is way too much (and detrimental to your fuel range/economy).
This along with the aforementioned fuel supply and battery connection checks should cover your woes.
