My 91 truck runs too cold
 

3.0L V6
My temp guage will go up to about 1/2 but then gradually decrease in temp while moving. I suspected a stuck thermostat and changed it. Still the same. I have good heat coming from the heater. The idle is up and down & I suspect because of the temp changing. TPS has also been changed. Any ideas?
I'm in Ontario and temperature lately has been about 20-30 F.
thanks

I wouldn't think 20's would be cold enough to keep your engine from heating up, however a simple solution may be to cut some cardboard and cover over the radiator.
Another thing worth checking out would be: the FAN. I have come across vehicles that have been modified by previous owners who set the radiator fan up to always be on. Check the fan......good luck, sorry about my random probably un-insightful ideas.