Yeah but whats the outside temperature been in Rockport these days??? .......figure whenever the guage reachest its highest point, then the radiator flow is shut down by thermostat..... and when it dips dramatically, a sudden surge of cooled water is going through the galleries when thermostat opened......... and uh, coolant travelling through various passages will carry eddies of cooler temp coolant as thermostat does its job.

Otherwise, try blocking the radiator with cardboard.... this *might* have an effect at stabilizing fluctuations on that dangerously erratic (finely tuned?) temperture guage you are worried about.