coolant temperature drop

My 2006 GMC Savana van is running fine. After warming up it never goes above normal running temp. Without any noise or event the temperature gauge just drops to about half and stays there. Sometimes it will pop back up to normal and never over normal. I can burn my hands with the heater and there are no bubbles that I can find in the Coolant reservoir. What might BE the problem?
Kevin

Check the actual engine running tempt.
A digital no contact thermometer works good for checking.

My previous van, an '86 Econoline, did that. A new thermostat was the cure.
FWIW
YMMV