fluctuating temp gauge

[markm]

Any explanation for wide variations on temp gauge, from 210 down to 160. Sometimes it will stay 160 for 10 or 15 minutes before returning to normal temp. Occurs mostly in summer months, on hills with AC on. No messages given from message center. Antifreeze is at proper level.
Occurs independant of clutch fan engaging. Also seems to fast idle when first starting up after temp fluctuation has occurred.
Truck is 2002 2500 HD Dmax/Allison with 45K miles.

[redrider#1]

Possibly a sticking thermostat and/or a thermostat rated at a lower temp. than stock. My last truck was a 1997 Dodge with the Cummins engine and it constantly cycled from 160 to 200 back and forth, and that is just how the thermostat worked on it as I never made any changes to correct it and had no problems.