The first 3 high flow stats that I installed came from 3 different sources and all 3 failed in a short time. The replacement high flows have been fine. All Robert Shaw.
Thermostats do not just fully open and fully close instantly, but partially open to the degree necessary to maintain the temperature.
With the fluctuation you have, I would consider getting another high flow thermostat.
Also 2 small holes drilled into the stat allow a small flow all the time and helps purge air when filling the system. Check out Stewart or Moroso for pictures.