If the ambient temp is the same for both instances then I would be checking the thermostat to see if it is faulty. It may not have one though as I have had 3 A series here in Aussie and all had the thermos removed when I got them. If your radiator is in good shape then you may have impellor probs on your water pump. I'm running a 3 core on my A 14 and the thermo fan rarely comes even though I'm now running a thermostat so standard set up shouldn't give you any troubles.