It might be related to the cable being sticky or the housing being crimped. If you fold your Downtube often, then perhaps the cable is a little compromised. It looks like it's adjusted perfectly. I'd take the cable out of the housing, clean it well, check the housing carefully for crimps, then use some tri-flo (teflon lubricant) on the cable with a squirt into the housing before putting it all back together. I believe SA uses spiral housing, which tends to compress a little, so it's not quite as accurate as the parallel housing used for indexed derailleurs. Maybe change to new SIS cable housing if all else fails?