Quite possibly. Its always worth checking the heavy leads at all their ends.
I don't know how practical is it (and this does not always work) but if you get a jump lead from the battery earth to the gearbox (so in effect you're supplementing the existing wire) if there is *any* sign of improvement then its also a sign.
Having said that and seen the lead I'd have to suggest repair / replace it. Check the 'body' earth as well , I think its in front or around the battery tray area on the later 306's.
Andy.
91 205D-Turbo, gone but still missed
02 106D, TUD5B, gone but not really missed apart from the MPG