Heat us a interesting beast. The ambient temp will have a huge impact on temps. Typically racks do best in STABLE warm room temps (78-80ºF) the other issue is probe placement. Most will place the probe on the flexwatt and regulate the 'output' temp this is not going to be the same as the temp inside the tub. Typically as the ambient temp changes the interior tub temp will also change. (why stable room temps are needed) Add that heat rises so the top slots often run warmer than the lower spots. The cool room will compound this and the 'drop' will be greater.
Solutions... I'd replace the T-stat with something a bit more reliable and easily set. My first choice would be herpstat but that is me. The second thing and easiest solution is to regulate the room with the rack to 80ºF and keep it there. It is possiable to regulate a rack in cooler room temps but that is a much bigger job. I have my rack in a cool room and have thoughts on that but the best advise is to keep the room warmer and be done with it.