Are you using solid bottom boards and telscopic hives?
Sounds like the age old death due to excessive condensation inside the hive. If the hive did not have a small vent above the frames and air acess from below to circulate the air, moving the condensation out the vent, then it solidifies on the under side of inner top, rains on the bees, and kills them during cold weather. Periodic warmer periods can erase the evidence. If you've ever opened your hive and hoticed the moisture, that hive has a venting problem where the condensation build up can affect the bees.