Actually, I'm afraid it's rather Windows' implementation of SMB that sucks. I used to get that kind of error as well.
You should try to mount the NAS drive into a folder which is on the local drive; at least that would get you rid of the issue with the disappearing net path, maybe.
Sure enough if the drive goes missing for a while, as far as Windows in concerned, Bitcomet might still balk but perhaps it will resume (I don't know how exactly it works internally, and I've never tested this particular aspect nor can I at this moment).