Buderus Logamatic 2107 odd intermittent low room temperature problem

I have a Buderus boiler with a 2107 controller and outdoor temperature sensor. No room temperature sensors. The house is divided into two zones (one upstairs, one downstairs) with the zone valves controlled by two Nest thermostats, but all of the learning, etc...functions are turned off. Essentially they are just thermostats with WiFi capability. Boiler program is set at 66 from 4:00 a.m. to 8:30 p.m. and 63 from 8:30 p.m. to 4:00 a.m. daily. Thermostats are programmed to match (daytime setpoint is 68, nighttime setpoint is 63). Basically the function of having manual control thermostats is either to keep the zone valves open on grey/cloudy winter days when the house is not getting much heat gain from the sun or to keep the zone valves closed on sunny winter days when the house tends to heat up a lot from the many windows on the south wall.

This setup has worked pretty flawlessly for the past three years with the house remaining comfortable in all conditions. However, lately it seems that something is intermittently amiss with with either the controller, the thermostats, or maybe the outdoor temperature sensor? Twice in the last two weeks after leaving the house around 7:00 a.m. with the house at a very comfortable 68 degrees we've returned to the house (around 3:00 p.m.) at around 62-63 with the setpoint on both thermostats still at 68 (as they should be), both times I checked the boiler controller to verify that the proper program was running, that it was on the right day, the right time, that the temperature setpoints were correct, etc... Everything looked completely normal except that the actual boiler temps were low (around 105-100 degrees) whereas they would normally be 125-130 at least in this kind of weather (high 30s/low 40s outside). Finding nothing really "wrong" we just decided to wait and see if it would warm up some and just bundled up. Both times we woke up to the house (around 5:30 a.m.) at its usual 68 degrees.

Seeing nothing obvious wrong with the thermostats or the controller, is maybe my outdoor temperature sensor going bad? Is there any easy way test it? The sensor is on the north side of the house near the base of the foundation so its not getting "fooled" by heat from the direct sun or any other source. I suppose I could just replace it and see what happens though I haven't priced them out yet.